That's not very much sleep.

If you want a good nights sleep then you need to finish whole REM cycles. You know that you can sleep for only 90mins, but if you do it right, you can feel totally refreshed?

Not as good as about 8 hours sleep, but as long as you start and finish REM cycles then you are getting good sleep. They are about 90mins long I think so you can judge it to suit you.
You could actually wake up earlier than you normally do and feel better in the morning. You will still be sleepy when you wake up, but you won't be groggy after you get up.

As for LDing, the more sleep you get, the better, as long as you don't oversleep one morning- you won't feel tired enough at bedtime, and this affects when you wake up then next morning, and so on.