Last year,there was a time I also is only sleep 6 hours a day ,but the duration is not long,about 2 months. Everyone's sleep quality is different, so the time he\she spends on sleeping is different also.But I personally think that we still need to ensure that 7 hours of sleep a day, in order to better work the next day.I work from 9 am to 6 PM, but I'm a newcomer now,so I work overtime every day, sometimes added to 10 PM.When I'm tired, I don't want to wake up the next day,but i have to.If I didn't sleep well at night, the second day, I easy to get wrong,absence of mind at the work time.On the holidaies,I can sleep 12 hours a day. :)