Growth Hormone: The Rhythm of Growth and Recovery
Growth hormone (GH) is best known for spurring childhood growth, but it also plays an important lifelong role in metabolism and tissue repair.
In adults, GH helps maintain muscle mass, bone strength, and a healthy balance between fat and lean tissue by stimulating protein synthesis and fat breakdown. It’s often called an anabolic hormone, essentially a build-up hormone, and it even influences how we regulate blood sugar, working in concert (and sometimes in opposition) with hormones like insulin.¹ But GH’s actions don’t happen uniformly at all hours; they’re tied to our biological clock. As previous posts have explored the daily patterns of cortisol, melatonin, and insulin, we will now examine where GH fits into this circadian orchestra. We’ll see how GH secretion pulses follow a 24-hour rhythm, peaking during the night.
The basics of Growth Hormone
GH is produced by the pituitary gland and released in pulses throughout the day. Its primary job, as its name suggests, is to drive growth. In children and adolescents, GH spurs the growth of bones and tissues, enabling the dramatic height and developmental gains of youth. In adults, it remains vital for maintenance: GH supports muscle protein synthesis (helping repair tissues and build muscle), promotes bone remodelling and density, and encourages the body to tap into fat stores for energy (by stimulating lipolysis, the breakdown of fat).¹ GH also has important metabolic effects: it raises blood glucose levels and tends to counteract insulin’s actions (for example, by making muscle and fat cells a bit less sensitive to insulin). This might sound undesirable, but it’s actually part of a healthy daily balance. GH helps prevent blood sugar from dropping too low during overnight fasting, whereas insulin’s job is to lower blood sugar after we eat. In essence, GH is a master regulator of growth but also a key player in metabolism, coordinating with other hormones to keep our body in balance.
Growth Hormone’s Rhythm
GH is released in a pulsatile manner, with brief spikes of high secretion. Crucially, these spikes are not random; they follow a daily rhythm intimately linked to our central circadian clock. Under normal conditions, the biggest burst of GH hits shortly after we fall asleep at night. In fact, roughly 60–70% of our total daily GH secretion occurs during early sleep. GH levels surge during these initial hours of sleep and then taper off in the later half of the night. Smaller GH pulses do occur in the daytime, but they are comparatively minor; the hormone’s profile is essentially “low by day, high by night”. Biologically, this makes sense: nighttime is when the body turns to repair and build mode (fuelled by GH), whereas daytime is for action and feeding, when growth processes can be put on hold.
Why does GH peak during sleep? The coordination between GH and sleep is orchestrated by both the circadian clock and the act of sleeping itself. The brain’s master clock (SCN) helps program the general timing; it “knows” that night is the appropriate time for growth and repair hormones to rise. However, it turns out GH release is profoundly sleep-dependent: when we enter deep slow-wave sleep, neurons in the hypothalamus release growth hormone-releasing hormone (GHRH) and dial back somatostatin (a GH inhibitor), triggering the pituitary to flood the bloodstream with GH. If deep sleep is interrupted or doesn’t occur, that big GH pulse is blunted
GH does not work in isolation, it’s part of an intricate hormonal timing system. In the circadian hormone orchestra, GH’s nightly rise is complemented by the other hormones. For example, cortisol (the adrenal “stress” hormone) has an opposite schedule to GH. During the first half of a normal night’s sleep, cortisol is held to low levels, which coincides with GH reaching its highest levels. Then, as dawn approaches, GH output subsides and cortisol begins to climb, helping to wake us up and mobilize energy for the day. This relationship is thought to be adaptive: when GH is doing its tissue-building and fat-burning work at night, it’s good to keep cortisol (which is catabolic and breaks down tissue under stress) at bay; come morning, cortisol surges to provide alertness and glucose, while GH takes a back seat. GH also has an interplay with insulin. During the daytime, especially after meals, insulin is high and GH is relatively low. But at night, the balance shifts – GH helps raise blood sugar and free fatty acids, essentially counter-regulating the prolonged fasting period of sleep. In fact, the surge of GH in the late night/early morning hours contributes to the dawn phenomenon, a natural rise in blood glucose toward morning. GH reduces our insulin sensitivity in those pre-dawn hours, meaning the body’s tissues don’t respond to insulin as strongly, which leads to a mild rise in circulating glucose by morning. In healthy individuals, this is a normal adjustment (ensuring you have fuel available when you wake). The point is, timing is everything, and growth hormone’s nightly flourish is a perfect example of this coordinated timing.
Why this matters
The story of GH underscores how tightly our biology ties growth and metabolism to the 24-hour day. When this timing system is disrupted, the consequences ripple.
Over time, losing the synchronization between GH and the rest of the circadian orchestra can have metabolic consequences. Blunting or mis-timing GH means missing out on its benefits for tissue repair and fat metabolism. People who chronically get too little sleep, or whose sleep is at the “wrong” circadian time, often show changes in body composition and metabolic health. Low nighttime GH activity is one suspected factor linking poor sleep to weight gain. For example, adults with abnormally low GH levels (clinically GH-deficient due to pituitary disorders) tend to accumulate more body fat and less muscle mass if untreated. Even in healthy individuals, studies have noted that sleep deprivation is associated with a reduction in GH and an increase in counter-hormones like cortisol, a combination that has been linked to greater risks of obesity and metabolic syndrome. It’s not that GH is solely responsible for weight, but a disrupted GH rhythm can tilt the balance toward fat storage and away from optimal muscle maintenance. Furthermore, if GH surges occur at odd times (for instance, a shift worker releasing GH during their night shift when they’re active and perhaps eating), it could induce insulin resistance at those times when insulin is needed, potentially contributing to blood sugar problems.
Unlike some hormones that are hard to influence, GH will quickly cooperate and get back in line. Prioritizing consistent, high-quality sleep is step one. Ensuring you go to bed and wake up at about the same times each allows GH to surge as intended. Regular exercise is beneficial too; aside from its myriad health benefits, it can stimulate GH release and improve sleep quality.²
The take-home: minor changes in our behaviours will help our bodies grow stronger and healthier.
Dr. Jonathan Moustakis
Co-founder and CTO of Lume Health
References:
Vijayakumar A, Yakar S, Leroith D. The intricate role of growth hormone in metabolism. Front Endocrinol (Lausanne). 2011 Sep 27;2:32. doi: 10.3389/fendo.2011.00032. PMID: 22654802; PMCID: PMC3356038.
Kanaley JA, Weltman JY, Pieper KS, Weltman A, Hartman ML. Cortisol and growth hormone responses to exercise at different times of day. J Clin Endocrinol Metab. 2001 Jun;86(6):2881-9. doi: 10.1210/jcem.86.6.7566. PMID: 11397904.
