You’ve likely heard of human growth hormone (HGH), a hormone that fuels muscle growth and recovery. Well, this hormone is released during sleep, particularly during deep sleep.
This is a good brief info about sleep and muscle growth link
with the references at the bottom.
Usually, you fall into REM sleep 90 minutes after you fall asleep.
Since leucine and growth hormone are the key for protein synthesis and they are released during REM sleep which takes about 1.5-2 hours (Adults can spend up to 20% of their sleep in the REM stage) 2x4hours would also be an option. It looks like at least 3-3.5 hours straight sleep is a must.
"Growth hormone promotes growth and repair of muscle tissue, so HGH release during sleep is crucial for muscle building.
Growth hormone also plays a role in carbohydrate metabolism, affecting energy supply to muscle. Sleep deprivation may slow muscle growth by altering supply of energy substrate to muscle.
If deep sleep is interrupted, the release of growth hormone is halted. Thus, a good night’s rest is crucial for growth hormone’s benefits to take full effect."