Try this experiment.

If you have to get up to go to work or get the kids ready for school, then count backwards 7.5 hours. For example, if you have to get up at 6am, then go to bed at 10:30 pm. Do this for seven straight days…even on the weekends or days off. By the seventh day, if you are waking up without an alarm at 6am, then you know you need 7.5 hours of sleep.

Sleep Tip #269: Try this experiment.