Sleep is the most underappreciated health intervention available. Not because it's neglected in wellness discourse — it's not, everyone knows they should sleep more — but because most people still treat it as a passive, low-activity period when nothing important is happening. This is entirely wrong.
Sleep is when your hormonal repair cycles run. Growth hormone is released almost entirely during deep sleep. Cortisol regulation resets overnight. Insulin sensitivity is restored. Leptin and ghrelin — the hormones governing hunger and satiety — are calibrated. Reproductive hormones depend on undisrupted deep sleep to function properly.
Short or poor quality sleep is not just tiredness. It's a systemic hormonal disruption.
◈Key Facts
The hormones that sleep regulates
Growth hormone
Human growth hormone (HGH) does more than its name implies. In adults, it governs muscle repair, fat metabolism, cellular regeneration, and tissue maintenance. Roughly 70-80% of daily HGH secretion happens in the first deep sleep cycle — usually within the first 1-2 hours of sleep onset.
If you're not getting deep sleep — whether from insufficient total sleep, late sleep timing, alcohol, or other disruptions — you're shortchanging your HGH output. This affects body composition, recovery from training, skin quality, and long-term metabolic health.
Cortisol and the HPA axis
Cortisol follows a circadian rhythm that is calibrated during sleep. If sleep is short, fragmented, or poorly timed, the cortisol awakening response (the natural morning spike) is dysregulated. People with poor sleep consistently show altered cortisol patterns — often elevated in the evening when it should be declining, and dysregulated in the morning.
High evening cortisol is both a cause and consequence of poor sleep — another feedback loop that gets entrenched with chronic sleep deprivation.
Insulin sensitivity
Sleep deprivation reduces insulin sensitivity. A landmark study found that just one week of sleep restriction to 5.5 hours reduced insulin sensitivity by ~30% — comparable to gaining 8-13kg of body weight in terms of metabolic impact. This is one of the mechanisms by which chronic short sleep drives weight gain and metabolic dysfunction.
Leptin and ghrelin
Leptin signals satiety; ghrelin signals hunger. Sleep deprivation reduces leptin and raises ghrelin — making you hungrier and less able to register fullness. Studies consistently show increased caloric intake the day following short sleep, with preference specifically for high-calorie, high-carbohydrate foods. This is not a willpower failure. It's a hormonal response.
Reproductive hormones
Deep sleep is also when pulses of GnRH (the hormone that triggers LH and FSH, which in turn regulate ovulation and testosterone production) occur. Insufficient sleep disrupts reproductive hormone secretion. Women with chronically poor sleep show altered LH patterns. Male testosterone production, which peaks in the morning and depends on REM sleep cycles, is markedly reduced in sleep-deprived men.
What disrupts sleep architecture
Alcohol is particularly damaging in a non-obvious way. It helps you fall asleep but fragments sleep architecture — suppressing REM sleep in the first half of the night, then producing a rebound effect with more fragmented light sleep in the second half. Drinking even 1-2 units within a few hours of sleep consistently reduces sleep quality on objective measures even when people report sleeping fine.
Caffeine's half-life is 5-7 hours in most people. A coffee at 2pm still has 50% of its caffeine active at 9pm. It doesn't necessarily prevent sleep onset but reduces slow-wave (deep) sleep.
Late eating raises core body temperature and stimulates digestion at a time when both should be declining. Insulin secretion late at night conflicts with the circadian drop in insulin sensitivity that normally occurs in the evening.
Irregular sleep timing — varying your sleep and wake times by more than 30-60 minutes disrupts social jetlag, the misalignment between your biological clock and your social schedule. Consistent timing is as important as duration.
A £400 supplement stack won't compensate for chronically broken sleep. Hormonal optimisation starts with a sleep schedule, not a product.