Overall, GH concentrations were suppressed during the rhIGF-I infusion by 85±3%, mainly by attenuating spontaneous GH pulse amplitude (77±4% suppression).
The apparent GH pulse frequency was attenuated from 7.8±0.9 to 4.7±0.6 pulses/24h (P= 0.006).
IGF-1 is the primary feedback, and the mechanism is similar to what you see with the HPTA axis.
When there is enough Testosterone and Estrogen, your body has this mechanism in place to prevent you from making more.
The same thing happens with exogenous GH use.
When you administer exogenous GH, IGF-1 elevates.
When your body recognizes that IGF-1 is elevated, it starts to suppress GH pulse amplitude in a dose dependent manner.
If you are using supraphysiological amounts of GH for bodybuilding purposes, you can assume that your natural secretion is greatly inhibited during sleep as this negative feedback loop will prevent your body from secreting GH in your sleep like it normally would.
When endogenous GH production is suppressed, administering exogenous GH at times that lipolysis or muscle growth can be optimized is the most logical approach to injection timing.
If fat loss is the goal, then taking GH in a fasted state prior to fasted cardio would be ideal.
There is a ceiling on how much GH is needed to maximize lipolysis, so if your dosage exceeds 2 IU per day, then I would advise splitting up that dose into two (or more if needed) different periods of the day with as much time between them as possible.
If muscle growth is the goal, GH should still be administered in a pulsatile fashion to mimic endogenous secretions.
Topped out stimulation of autocrine IGF-1 in skeletal muscle can be achieved with fairly low dosages of exogenous GH, and administering a higher dose above and beyond that may just be a waste.
This is why it would be wise to split up your daily GH dose into multiple administrations if you are using supraphysiological dosages.