>>65450135If your sleep cycles are getting disrupted, you may wake up mid-cycle full of sleep hormones and feel like shit, but you may also not be getting a meaningful amount of REM sleep in the process since successive cycles give more. If you are missing a lot of REM sleep, your body will compensate with longer REM sleep, which can also make you feel like shit but more in the sense of a confused stupor ("feeling depressed"). You might also not be sleeping at all if you have undiagnosed sleep apnea or drink a lot before bed (REM sleep won't occur until you sober up), which means you're both in debt and being assaulted by rebound.
Sleep cycles typically last about 90 minutes and you can only wake up feeling fully refreshed if you're in-between cycles or close enough to it. Exactly 8 hours of sleep isn't 7.5 hours or 9 hours (multiples of 90 minutes), which may contribute to your issue if you are actually timing this out and waking up with an alarm.