Nocturnal GH Pulse

The nocturnal pulse is the single largest growth hormone secretory event of the day in healthy young adults. It begins within roughly the first hour of sleep and is locked not to clock time but to slow-wave sleep: shift sleep to the daytime and the pulse follows it, and suppressing deep sleep suppresses the burst even when sleep duration is preserved. Mechanistically it reflects a withdrawal of somatostatin tone coinciding with GHRH drive, which is why the pulse is so much larger than daytime bursts of similar trigger.

In young men most daily growth hormone output is sleep-associated, a fraction high enough that a disrupted night measurably lowers the 24-hour total. Both slow-wave sleep and the nocturnal pulse decline steeply from the twenties onward, and they decline together, which is the main reason ageing is accompanied by lower growth hormone without any pituitary lesion. Women show a flatter profile with higher interpulse concentrations and less dominance by the sleep pulse.

This matters for interpretation more than for intervention. It explains why sampling time destroys or preserves the information in a growth hormone measurement, why sleep-disordered breathing confounds axis assessment, and why an age-related fall in growth hormone is better read as a normal trajectory than as a deficiency state requiring correction.

The inference that does not follow is the marketing one: that because deep sleep drives growth hormone, any compound that raises growth hormone improves sleep, or that timing an injection at bedtime meaningfully reproduces physiology. Causality runs from slow-wave sleep to secretion, and evidence that exogenous growth hormone or secretagogues improve sleep architecture is thin and inconsistent, with some agents in this class reported to alter sleep stages rather than deepen them.