Sleep, hormones and regeneration: the quietest lever in the system

Week three of your new eating window. Training is going well, and during the day you actually feel good.

And still you lie wide awake at half past three, as if someone had switched on a light inside you.

Many people know this pattern. Others know the flip side: one week of shift work, and suddenly there are cravings that do not feel like you.

Sleep is the quietest lever in the whole system. It makes no noise, but at night hormone axes are set that can help decide your hunger, mood, performance and cycle during the day.

Here we take a close look: which hormones work at night, what short nights shift, what studies show about muscle, immune defence and the brain, and why sleep can grow quieter early when energy is clearly lacking. Where the data are thin, I will tell you.

The hormonal night: which hormones work during sleep

You are asleep. Your hormone system is on shift.

Imagine the night as a shift schedule in a workshop. Deep sleep in the first half of the night is the time for building up, while the REM phases towards morning are more about handover and sorting. The picture simplifies, but it shows why a fragmented night can throw several axes out of rhythm at once.

Growth hormone: the pulse at the start of deep sleep

Growth hormone is a building and mobilizing signal. It is involved in releasing fat as fuel, maintaining tissue and triggering IGF-1 in the liver, the messenger through which part of its building effect runs.

This fits with an analysis by Van Cauter and colleagues of 149 healthy men. The share of deep slow wave sleep fell from 18.9 percent at ages 16 to 25 to 3.4 percent at ages 36 to 50, and growth hormone secretion declined markedly in parallel. Cross-sectional data, men only, an association and not proof.

A finding by Brandenberger and colleagues takes the edge off a widespread myth: after sleep deprivation the nocturnal pulse was missing, but it was made up during the day, and the 24-hour amount stayed similar. So a sleepless night does not wipe out your growth hormone. This short report says nothing about chronically short sleep.

Cortisol: a quiet zone in the first half of the night, a start signal in the morning

Cortisol is not an enemy. Among other things, it is the timekeeper that gets you ready to go in the morning.

Towards morning the picture turns. The cortisol awakening response, a marked rise in the first 30 to 45 minutes after waking, is normal and desirable. An expert panel led by Stalder stresses that measuring it depends on taking the saliva samples exactly from the moment of waking. A saliva test with approximate times therefore says little.

Why you can be wide awake at three in the morning is explained in Awake at 3 am, and how morning light sets the start signal in Chronobiology: morning light and the internal clock.

Melatonin: the signal of darkness

Melatonin is not a classic sleeping pill but a message: it is dark outside. Light slows this message down.

Melatonin as a supplement or medicine has its own guideline position, which you will find further down. I deliberately do not give a dose; the context is in Melatonin: effects, dosage and myths.

TSH: a rise in the evening, a brake from sleep

The thyroid axis also keeps time. Allan and Czeisler observed that TSH rises in the four to five hours before sleep and drops steeply with falling asleep. When participants stayed awake, TSH remained elevated all night. After six nights with only four hours in bed, however, TSH was lower in another study with eleven young men: acute sleep deprivation and chronic sleep debt are not the same thing.

So the time of the blood draw is part of the TSH value. I do not give reference ranges because they depend on the laboratory. More in Which thyroid values matter.

Prolactin: a sleep hormone in the literal sense

You may know prolactin only from breastfeeding. In ten young healthy men, a study led by Spiegel found that deep sleep activity in the EEG and prolactin secretion ran in parallel, and in nine out of ten the coupling to delta waves was significant. So prolactin, like growth hormone, seems to follow the rhythm of deep sleep, and a prolactin value depends on when blood is drawn. More in Prolactin and the thyroid in men.

Testosterone: a rise with falling asleep, a plateau by the first REM phase

Luboshitzky and colleagues took measurements every 15 minutes from 7 pm to 7 am in six young men. Testosterone was lowest while awake in the evening, began to rise with falling asleep, reached its peak around the first REM phase and stayed at this level until waking.

When ten men slept in a fragmented way in a second study, the total amount stayed similar, but the rise shifted markedly later and appeared only in the four men with REM phases. So the architecture of the night seems to count as well.

Supported by human laboratory studies, mostly small and mostly in young men. The direction is well described, but the magnitude in women and older people has been studied much less.

And now you know why a shifted night does more than make you tired.

What a short night shifts: cortisol, hunger, insulin, testosterone

After a short night, you reach for a second croissant at eleven. Lack of willpower?

Maybe not. Figures like cortisol up 37 percent or ghrelin up 28 percent circulate online, almost never with a source and study size. Let us make up for that here.

Spiegel, Leproult and Van Cauter had eleven young men spend only four hours in bed for six nights, followed by twelve hours in bed for six nights. With sleep debt, glucose tolerance was poorer, TSH lower, evening cortisol higher and the sympathetic nervous system more active. The order was fixed, not randomized.

Where the 37 percent for cortisol comes from

The often quoted cortisol figure comes from a study by Leproult and colleagues. Young men, whose exact number the study abstract does not state, slept normally, slept only from 4 am to 8 am, or stayed awake all night. On the evening of the following day, between 6 pm and 11 pm, cortisol after the shortened night was 37 percent higher than on the previous day, and after the sleepless night 45 percent higher. The evening cortisol quiet phase began at least one hour later. So the figure describes evening cortisol after a bad night, not a permanent state. But it does explain why one bad night can carry over into the next.

Hunger: what the hormones say and where the studies contradict each other

Insulin: even one night is measurable

Insulin is the key that moves sugar from the blood into the cells.

Not only how long, but when

Scheer and colleagues shifted sleeping and eating by about twelve hours against the internal clock in ten adults in the laboratory. Leptin fell by 17 percent, glucose rose by 6 percent despite 22 percent more insulin, the cortisol rhythm was completely reversed, and three of eight participants with sufficient data showed post-meal glucose levels in the prediabetic range. An extreme situation, similar to demanding shift work, but it suggests that timing can be a lever of its own.

Testosterone: a single finding and a meta-analysis

Leproult and Van Cauter had ten healthy young men spend only five hours in bed for eight nights after three nights with ten hours in bed. Afterwards, daytime testosterone was 10 to 15 percent lower, in a small convenience sample, as the authors write.

A meta-analysis by Su and colleagues with 18 studies and 252 healthy men puts this into context: short-term restricted sleep had no significant pooled effect on testosterone, whereas total sleep deprivation lowered it. So the single study cannot be generalized. More in Cortisol, stress, sleep and testosterone.

Supported by small controlled laboratory studies and meta-analyses. The direction for cortisol, ghrelin and insulin is similar across several studies, while for leptin and for testosterone after merely shortened sleep it is inconsistent.

And now you know why the night helps decide how the next day feels.

Sleep and hormones in women: cycle, progesterone, perimenopause

The same two or three restless nights every month. Or no sleeping through the night for a year, even though little else has changed.

First things first: most of the studies above come from young men, and much less has been studied in women.

The cycle: more stable than its reputation, but not for everyone

A review by Baker and Lee adds that self-reported sleep quality worsens in some women when premenstrual symptoms set in, and not in others. Women with PCOS have an increased risk of sleep-related breathing disorders, which should be treated, see Recognizing sleep apnea.

Progesterone: calming, but thinly supported

Progesterone is credited with a calming effect via the GABA system, the braking system that many sleeping pills also act on. Mechanistically plausible, thinly supported in humans. In a randomized, double-blind crossover study, Schüssler and colleagues examined ten healthy postmenopausal women who each received oral micronized progesterone for 21 days (study dose 300 milligrams). Nighttime wake time decreased, and cognitive performance remained unchanged.

The dose is a study figure, not a recommendation. Progesterone is prescription-only, and whether hormone treatment may be an option belongs in a gynecological consultation. More in Progesterone deficiency: symptoms and testing.

Perimenopause: the stage counts, not age

In the American SWAN study, 38 percent of 12,603 women aged 40 to 55 reported sleep problems. Age-adjusted rates were highest in late perimenopause at 45.4 percent and after surgical menopause at 47.6 percent. Age alone was not significantly associated with sleep problems.

In the longitudinal analysis with 3,045 women, more frequent hot flashes were associated with more sleep problems of every kind, and falling estradiol with more trouble falling and staying asleep. Observational data, not proof of cause and effect.

A meta-analysis by Cintron and colleagues pooled seven randomized trials of hormone therapy with 15,468 women. Sleep quality improved in women with hot flashes, while no difference was seen in women without hot flashes, at a moderate to high risk of bias.

The German S3 guideline on insomnia notes that insomnia occurs more frequently in connection with menopause and refers to the separate S3 guideline on peri- and postmenopause. Treatment options are covered in Menopause and sleep disorders, the start of the transition in Perimenopause: symptoms and when it starts and the influence of stress in Cortisol, stress and female hormones. Hormone therapy or hormonal contraception should not be started, paused or stopped on your own because of a sleep problem.

And now you know why a general rule about women and sleep falls short.

Regeneration in the body: muscle and immune system

Training is going well, your protein intake is right, and still you plateau. Or it is your third cold this winter, and you wonder what you are doing wrong.

Regeneration sounds like doing nothing. In the body, it means building, remodeling and sorting.

Muscle: what a sleepless night shows the next day

The growth hormone myth, put into context twice

Two fitness claims fall short: no deep sleep, no growth hormone, and fasting boosts growth hormone. The first is softened by the daytime compensation after a sleepless night, the second by a study by Ho and colleagues in six healthy men. Five days of fasting increased the number of growth hormone pulses from 5.8 to 9.9 per 24 hours, while IGF-1 (measured as somatomedin C at the time) fell from 1.31 to 0.77 units per milliliter between the first and fifth day of fasting.

The mobilizing signal rose, the building signal fell. And catching up on sleep? For growth hormone after one night, that seems to work, but for muscle, long-term data are lacking.

How recovery after training works overall is covered in Regeneration is not doing nothing, warning signs in Recognizing overtraining and the role of muscles in Strength training after 40.

Immune system: sleep in the night after vaccination

Prather and colleagues measured sleep in 164 healthy people for one week and then, under quarantine, gave them cold viruses into the nose. Compared with more than seven hours of sleep, the risk of a cold was increased with less than five hours (odds ratio 4.50) and also with five to six hours (odds ratio 4.24). Between six and seven hours, it was not significantly increased (odds ratio 1.66). The confidence intervals were very wide.

And inflammation? In a meta-analysis by Irwin and colleagues with 72 studies, disturbed sleep was associated with somewhat higher CRP and interleukin-6. Shorter sleep duration was linked only to a very small rise in CRP, not to higher interleukin-6 and not at the extreme of very short sleep, whereas very long sleep was linked to both markers. Experimental sleep deprivation changed none of the markers. A review by Besedovsky and colleagues sees persistent sleep loss as a possible route to low-grade inflammation and names the hormonal constellation of the night as a likely mechanism for supporting immune defence. Both views stand side by side.

Supported by small randomized studies and meta-analyses. For muscle and vaccine response, the direction is consistent, while long-term effects on muscle building and the transfer to women are open. The role of hormones as a bridge between sleep and immune defence is mechanistically plausible.

And now you know why the night after a hard day is part of training.

Regeneration in the brain: memory, brain fluid and an open debate

After a bad night, your head feels like cotton wool. Words come more slowly, and patience runs shorter.

Well supported: sleep and memory

Rasch and Born summarized more than a century of research in a large review. That sleep supports the retention of memories is considered established. During deep sleep, freshly learned material is reactivated and remodeled for long-term memory, and the REM sleep that follows could stabilize it.

Like in a library: during the day, books land on a pile, and at night they go onto the shelves.

The brain wash during sleep: two mouse studies, two results

What was measured in humans

Fultz and colleagues combined fast MRI imaging with EEG in sleeping people: during non-REM sleep, slow brain waves were followed by fluctuations in blood flow, and rhythmic waves of cerebrospinal fluid were coupled to these. That shows movement, not cleaning.

Shokri-Kojori and colleagues found an increased beta-amyloid signal in the right hippocampus and thalamus on PET in 20 healthy people after a sleepless night. This is no evidence that individual bad nights cause dementia, and nobody needs to fear for their brain after one sleepless night.

Even Nedergaard, senior author of the mouse study by Xie mentioned above, writes cautiously together with Hauglund in a 2026 review: in humans there are parallel signals, but better measurement methods are needed, and the role of clearance for restorative sleep is a hypothesis.

Mechanistically plausible, contradictory in animals, human studies small. If the fog in your head persists for weeks, there are many possible causes, sorted in Brain fog: recognizing the symptoms.

And now you know why I tell the story of the brain wash during sleep only with a question mark.

Why sleep can suffer early under energy deficiency

You eat less, train more and actually feel good. Only the nights grow shallower, and at four you are wide awake.

This chapter is the heart of the article: why sleep can grow quieter early when the body lacks energy, and when that does not happen.

Energy availability instead of counting calories

Energy availability does not only ask how much you eat, but how much energy is left after training for all the other tasks of the body: hormones, bones, immune defence, warmth and, yes, sleep. How this equation affects the cycle in women and how the reproductive axis responds is covered in Fasting for women: why the cycle matters.

In 2023, the International Olympic Committee published a consensus on this. Low energy availability, LEA for short, is there, in essence, any mismatch between energy from food and energy for training that leaves the body's total needs uncovered. The key point: the IOC describes LEA as a continuum.

• Adaptable LEA is typically a brief experience with minimal or no consequences for long-term health, wellbeing or performance.
• Problematic LEA is associated with more pronounced and possibly persistent disturbances of various body systems, often shows up with signs or symptoms and is considered maladaptive.

Problematic LEA can lead to REDs, formerly RED-S: a syndrome of impaired physical and psychological functioning in athletes. Primary indicators in the assessment tool include an absent period due to functional hypothalamic amenorrhea, low testosterone in men, low T3 and bone stress injuries.

Two points matter to me. The IOC tools apply to athletes, and transferring them to people who fast or diet without competitive sport is an obvious but untested inference. And energy deficiency is not measured by weight: people of normal weight can also have too little energy, because the definition is not tied to any body weight.

And where does sleep stand in this consensus? The IOC counts reduced sleep quality among the psychological indicators of problematic LEA, alongside mood swings, restrained eating, the drive for thinness and perfectionistic tendencies. In the assessment tool, however, sleep disturbances appear only among the potential indicators that are not yet scored. As differential diagnoses, the IOC names, among others, mental illness, shift work, obstructive sleep apnea, pain, medication and restless legs syndrome. So sleep is a possible early signal, not a diagnostic criterion.

What was measured in humans regarding sleep

At the far end of energy deficiency, the link has long been described. In 1975, Lacey and colleagues recorded the sleep of ten people with anorexia nervosa in inpatient treatment: at low weight they slept less and more restlessly, especially in the last four hours of the night, and with refeeding, sleep duration and REM sleep increased.

A new, very small report by Lundstrom and colleagues points in the same direction, with 16 female and 10 male swimmers in a hard training phase: higher energy availability was associated with more REM sleep and, after accounting for sex, with longer deep sleep. It was measured with a wearable whose sleep stages are only estimates (see Sleep trackers: how accurately they measure), and the authors themselves speak of preliminary evidence.

Mechanistically plausible, human studies thin: small studies and cross-sectional data, no randomized long-term studies.

When eating becomes a struggle

An article about energy deficiency and sleep can have a side effect: the urge to control eating and sleep even more closely. That is not the intention. Restrictive eating and a long history of dieting are themselves risk factors for disordered eating, and the line between discipline and compulsion is often hard to see from the inside.

Do your thoughts revolve around food, calories or weight for a large part of the day? Do you feel guilty when you have eaten more than planned? Do you train to compensate for food? Has your period stopped? Then that is a signal and not a failure. And a reason to seek medical or psychotherapeutic support, sooner rather than later.

What eating disorders involve and why they have nothing to do with a lack of willpower is described in Understanding eating disorders. That is why this article contains no calorie, weight or energy targets, only labeled study figures.

And now you know why the quietest lever can grow quieter early under energy deficiency.

Fasting, calorie deficit, intermittent fasting: when it can disturb sleep and when it does not

Some people sleep like a log when fasting, others lie awake. Both are true.

One week of modified fasting was also associated with fewer arousals and better rated sleep in a small, uncontrolled pilot study by Michalsen and colleagues.

There are two meta-analyses on time-restricted eating, one form of intermittent fasting. Jin and colleagues pooled 13 studies with 638 participants: before and after, sleep duration increased slightly, while compared with control groups there was no difference.

Lima and colleagues included 51 papers, including on Ramadan fasting. Pooled, time-restricted eating shortened self-reported sleep duration and worsened subjective sleep quality. With Ramadan fasting, where eating takes place at night, sleep duration was significantly shorter, while studies without Ramadan tended to show longer sleep and no difference in sleep quality compared with control groups. This is not a judgment of a religious practice, but a pointer to how much the timing of meals counts.

What decides whether fasting affects sleep

Laid side by side, the studies form a pattern. Whether fasting can disturb sleep seems to depend less on the method than on these factors:

• Severity of the deficit. Complete fasting raised cortisol markedly, moderate restriction did not.
• Duration. One day with little energy barely changed sleep, while four weeks with a marked deficit were associated with less deep sleep in a small study.
• Training load. A lot of training with little energy shifts the energy availability equation.
• Timing of meals. Eating at night during Ramadan was associated with shorter self-reported sleep in the meta-analysis, eating windows without Ramadan rather with longer sleep.
• Stress and life stage. Energy and the stress axis interlock, and in women the cycle and perimenopause add to this.

And in the study with 8.5 versus 5.5 hours of sleep opportunity, people who additionally cut back on sleep during a deficit lost more fat-free mass with the shorter opportunity. Sleep and energy are connected in both directions.

How women may respond to forms of fasting depending on their life stage is covered in Fasting for women and Intermittent fasting for women over 40. I deliberately do not recommend a fasting guide or a time window.

Supported by randomized studies and meta-analyses is that supervised moderate restriction and intermittent fasting without eating at night did not worsen sleep on average. Mechanistically plausible, human studies thin is that marked, persistent deficiency can disturb it.

And now you know why the same form of fasting can make two different nights for two different people.

What you can influence and when it should be checked by a doctor

You do not want yet another rule. You want to know which factor counts.

First things first: sleep disorders have many possible causes, and the map for them is in Treating sleep disorders holistically. Here it is about the conditions around hormones and energy, as directions, not as recipes.

Duration: the only number with the character of a recommendation

In a joint consensus, the American Academy of Sleep Medicine and the Sleep Research Society recommend that adults between 18 and 60 years of age regularly sleep 7 hours or more per night. Regularly sleeping less than 7 hours is associated there with adverse health outcomes, such as weight gain and obesity, diabetes, high blood pressure, heart disease and stroke, depression and a higher risk of death.

Associated means a connection, not a proven cause. And there are phases of life in which seven hours are simply not possible. Then it is not about perfection, but about what can be protected.

Rhythm and light

The laboratory study with shifted sleeping and eating showed how much a rhythm against the internal clock can shift. Regularity is an underrated lever, and a fixed wake-up time can give the internal clock an anchor.

Light is considered the strongest timekeeper of the master clock, and even room light in the evening can shorten the melatonin night. Among its sleep hygiene rules, the S3 guideline on insomnia lists avoiding bright, activating light before bedtime. More on morning and screen light in Blue light and the circadian rhythm.

Meal timing

Wehrens and colleagues shifted three meals five hours later in ten healthy young men in the laboratory. Sleep, melatonin, cortisol and hunger remained unchanged. The blood sugar rhythm, by contrast, shifted by 5.69 hours, and in adipose tissue the rhythm of a clock gene shifted by 0.97 hours.

So meal times seem mainly to set the peripheral clocks in metabolism, while light sets the master clock. The S3 guideline also lists: no heavy meals in the evening and no caffeinated drinks after lunch. That is consensus, not the result of large studies.

Training

A meta-analysis by Kredlow and colleagues with 66 studies found that regular exercise can improve total sleep time and sleep efficiency to a small extent, time to fall asleep to a small to moderate extent and sleep quality to a moderate extent. The S3 guideline states that exercise therapy can be considered for insomnia (recommendation grade 0).

And evening training? Stutz and colleagues found no support in 23 studies for the concern that it worsens sleep, if anything the opposite. Only intense training that ends one hour or less before bedtime might disturb falling asleep and sleep duration.

Energy availability: thinking of energy and sleep as a pair

If you train a lot, fast or eat less, watch for early signals: sleep, period, mood, performance, infections. None of them proves energy deficiency, but several together, at the same time as a diet or more training, are a piece of information.

The sleep stages of a tracker are estimates, and a morning with little deep sleep in the app is not a medical finding. And no evening drink and no over-the-counter supplement replaces rhythm and sufficient energy. Prescribed medications are explicitly not meant here.

When it should be checked by a doctor

For chronic insomnia, the guideline position is clear. The S3 guideline on insomnia states: cognitive behavioral therapy for insomnia shall be recommended as the first treatment option for all patients with insomnia, preferably in person (recommendation grade A). The European Insomnia Guideline 2023 likewise recommends it as the first treatment for adults of any age, in person or digitally (A). More in CBT-I and sleep restriction.

On sleeping pills, the S3 guideline states: benzodiazepines and related substances, which include the Z-drugs, are effective in short-term treatment of up to four weeks. Long-term treatment shall not be recommended (A). Among the side effects, it lists tolerance, dependence, falls and rebound insomnia after stopping. A review of discontinuation phenomena describes that withdrawal syndromes with substances such as benzodiazepines can be life-threatening, with seizures and delirium. That is why: if you have been taking such medications for a longer time, do not stop them abruptly, and any tapering belongs under medical supervision.

On melatonin, the S3 guideline states that it is effective for insomnia in people aged 55 and over and is approved in Germany for short-term treatment in this age group. Long-term treatment shall not be recommended (A). The European guideline considers prolonged-release melatonin from age 55 for up to three months an option (B) and does not recommend fast-release melatonin for insomnia (A). According to the S3 guideline, herbal medicines should not be recommended for the treatment of insomnia (B).

Stress and sleep interlock. Vgontzas and colleagues found significantly higher ACTH over 24 hours in eleven people with chronic insomnia compared with 13 healthy people. For cortisol, the difference was P = 0.07, just above the usual significance threshold, even though the authors describe both values as elevated. Guidance is offered by Burnout and sleep disorders and Sleep and depression.

If energy deficiency is suspected, an absent or irregular period, bone stress injuries, marked weight loss, a persistent drop in performance or signs of disordered eating should be checked by a doctor. According to the Endocrine Society, functional hypothalamic amenorrhea is a diagnosis of exclusion, and its complications include bone loss and infertility. Iron status and the thyroid should also be looked at, see Iron deficiency and the thyroid as sleep thieves.

Cortisol, TSH, prolactin and testosterone fluctuate across the day and night. A value without the time of day, without the night before and without the clinical picture says little, and reference ranges depend on the laboratory. Please do not interpret such values yourself or derive a treatment from them; discuss them with a doctor instead.

Clinical tradition without a strong study base is the integrative view of sleep as the first lever before any supplement. It does not replace diagnostics or prescribed treatment.

If you want to do more than read and have these connections looked at in your own case: below this article you will find the option to book an appointment.

And now you know why I call sleep the quietest lever in the whole system: it works quietly, it can grow quieter early under energy deficiency, and you can reset it every evening.

Frequently asked questions about sleep, hormones and regeneration

Where sleep connects to the rest of the body

Sleep rarely stands alone. It is tied to energy, the stress axis, the cycle, training and metabolism. These articles lead on from here.