Pulsatile release

Growth hormone is not released at a steady rate. The pituitary secretes it in pulses, several a day, with the largest during the first hours of deep sleep, and the level between pulses falls close to zero. The pattern is set by an interplay between GHRH, which drives pulses, and somatostatin, which suppresses them, with ghrelin adding a third input. Many of growth hormone's effects on tissue depend on the pulse pattern, not just on the total amount.

This is the argument the research literature makes for growth hormone secretagogues over growth hormone itself. A short-acting secretagogue such as ipamorelin or CJC-1295 without DAC triggers a pulse and is gone, leaving the natural rhythm and the feedback loop intact. Long-acting exposure, whether from injected growth hormone or from CJC-1295 with DAC, which keeps the GHRH receptor occupied for a week, produces continuous elevation instead. The two approaches are studied for different questions, and the profiles say which the cited studies used.

Timing follows from the biology. Research protocols for short-acting secretagogues frequently dose before sleep to coincide with the largest natural pulse, and the growth hormone secretagogues article explains why.