Copied from a similar question I answered on the fitness SE:
You mentioned being tired during the day, does this happen even when
you are getting enough sleep? You could have sleep apnea, a
condition which causes you to stop breathing in your sleep. Sleep
apnea constantly goes undiagnosed because sufferers don't realize that
it is not normal to be tired during the day.
I started treatment for sleep apnea earlier this year and I cam say
with no exaggeration that it has been life changing. No more falling
asleep behind the wheel, no more afternoon fatigue, etc. I don't know
if this is common, but I've also had an easier time falling asleep
since starting treatment.
Contrary to popular belief, you do not need to be overweight to have
sleep apnea, although it could be a sign. Sleep apnea tends to cause
weight gain just as much as weight gain will cause sleep apnea. If you
have a history of depression, snore, or if you are a mouth breather,
these are also strong indicators.
If you have a significant other ask them to pay attention to your
breathing patterns, if not ask your doctor to do a sleep study.
They're usually conducted in hotels, it's easy, comfortable, and won't
interrupt your schedule (you'd be in bed either way). When I did mine
it was right next to the office, so I got to avoid the morning
If there is a qualitative (as opposed to quantitative) difference between your tiredness during the day when getting 7 hours/night and how you feel when you haven't gotten enough sleep, this is a strong indicator as well. Talk to your doctor, most sleep conditions can be managed and your quality of life will improve drastically.