system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5: Why does the time you feel like you were asleep for not match how long you were actually asleep for?

There are times I can take a 15min. nap on my break during work and feel like I slept for an hour and be energized for the rest of the day. Yet there are times I can sleep for 8 or 9 hours wake up and feel like I just shut my eyes just couple seconds ago. Why does the mind do this and is there anything we can do to influence or control it?
gpt
Sleeping for a longer time (8-9 hours) isn't as important as quality of sleep. Sleep cycles are around 1.5 hours, and take about the same amount of time to fall into REM sleep - where all the good rebuilding and restoring happens. 

If you wake in the middle of one of these cycles, say at 8 hours, you're going to feel groggy and tired because your body wasn't in a great place to wake up yet. Sleeping for a super short time, like a 20 minute power nap, allows us to rest and shut down briefly without falling into that heavy sleep that's hard to wake from.

You can help this be more predictable by keeping a consistent sleep schedule and shooting for 90 minute intervals. Plan to get 4.5, 6, or 7.5 hours, (leaving time to fall asleep of course,) and base your alarm off of that. It's very tempting to milk every minute of sleep you can by snoozing your alarm or going back to sleep after you've woken up naturally just because you have the time to. Don't do that, trust me, it never helps.