The following is based on the assumption that you're not up to date on the latest scientific research on sleep cycles. If you are then feel free to ignore this.
Forget the classic "you need at least 8 hours of sleep every night" line you've been fed by nearly every person since you were born.
Fact of the matter is, we out minds and bodies don't sleep in cycles of 2s or 4s (hours), so 8 hours is not ideal at all. In fact, we sleep in cycles of 3 hours. You can go to sleep at 12am and wake up at 3am and feel completely refreshed and not be in any danger of sleep deprivation. I usually sleep or 6 hours each night, sometimes 9 on the weekends.
So I suggest setting an alarm and/or trying to get into the habit of sleeping in 3-6-9 hours every night/day. I promise you that you'll feel a thousand times better. The reason you feel exhausted is because when sleeping, your body/mind is concentrated on resting until it hits a 3 hour interval, at which point it can be pleasantly waked. However, if you wake in the middle of one of these intervals, such as 8 hours or 10 hours, your body/mind rejects to waking up violently and you feel exhausted, unfocused, etc throughout the day.
Also, should go without saying that the intervals don't need to be at exactly 3 hours before waking. I believe there's about a 20 min safety window more or less where you can still wake up feeling refreshed. For example, falling asleep at 12am and waking at 5:45am or 6:15am will still be refreshing sleep (possibly even 30 mins allowed).