6 to 8 hours is actually pretty normal for adults...older adults need even less. Teens need more than 8 hrs. But for adults, if it's that amount every night, that's decent.

The issue may be the time you're sleeping. I know you've been doing this for a long time. I worked third shift 4 nights a week for 5 years. I already had preexisting issues with mental illness (depression, specifically) but the third shift schedule really messed me up. I didn't experience my worst until after I'd been working thirds for 5 years, and I kind of took a nose dive. My doctor thinks it was because of the irregular sleep pattern (even if you're getting a good amount of hours, sleeping during the day is irregular--your circadian rhythm is off, hormones get disrupted, etc). That was over two and a half years ago. I took a two month leave of absence then went to first shift a few months after coming back to work, and I felt like a new person. I encourage you to discuss this with your doctor.

Also, if you don't have good black-out curtains in your bedroom, I suggest them because they are fantastic. I still have them up and I sleep normal hours.