I remember some military study concluding that there is no replacement for time spent asleep. If there's a time debt, the only cure is to repay the time debt, no shortcuts.
personal experience X a bit of personal research on the matter:
- deep sleep is restorative (AIUI the brain needs deep sleep otherwise it poisons itself, whereas e.g muscles don't, they need rest)
- light sleep is important too but I feel that was not the question
- I noticed I had a hard time falling asleep early/at "normal" hours
- even if I did, waking up before 9am made me feel terrible through the day until 4pm, even though I had 8-12h of sleep
- I got a fitness device (back then fitbit charge HR, now garmin forerunner 735xt) for unrelated reasons but figured out any data could be helpful as long as I'm aware of the limitations and error margins. Either device I had/have cannot record REM sleep, only deep+light+movement (via HR monitor+accelerometers)
- over five years, data shows that either I sleep no earlier than 1am, or when I sleep before, it's only light sleep and a lot of movement, deep sleep starting between 1-3am
- also, attempts at imposing myself an earlier sleep schedule (in order to get 8h of sleep given I started work at 9am) resulted in worse sleep/later deep sleep
- sometimes on a given night, cumulative deep sleep as reported may look to be enough (e.g 3-4h) but looking at that night timeline graph showed that it was extremely segmented instead of a couple or three chunks
- even if I'm extremely tired I cannot sleep before 1am. it's that or I sleep at 7-9pm and wake up within an hour after 23pm, theb can't get back to sleep until 3-4am
- exercise helps (1h a day), circadian lighting helps (via home assistant + ikea tradfri), reading before sleep helps, time outside helps, sex helps, but any of that does not offset my schedule, it only makes it so I get more/better deep sleep
- food doesn't seem to have much of an effect on me, except sugary stuff which makes me twitchy, and pasta which makes me sleepy but sleep quality is no better (and actually worse because it screws up my schedule)
- caffeine has a measurable effect when taken after 2pm, non measurable before that
- alcohol ruins my deep sleep with some linear-ish correlation
- I also tracked weight, with bad sleep I could eat very little and not lose extra weight, but as soon as I get good sleep over a long enough duration, I can shed extra weight quickly, irrespective of food or exercise
Overall, the device helped a lot in clearing some biases and understanding myself better, even though its data is imperfect and imprecise (e.g sometimes data is obviously messed up, sometimes it can't account for odd sleeping events) but being cognizant of that allowed me to interpret said data, understand specific events and long term trends, and manage my sleep schedule.
EDIT: scratch the 5 years, I just checked and it's actually 8 years, with some gaps. goddammit time flies.