Over the years I've found that I definitely need 8 hours sleep but didn't often get it. I would normally manage about 6-7 hours and would soon build up a deficit. This had a definite impact on my well being and performance at work. Recently I've started using a timer instead of an alarm clock to ensure that I always get eight hours. If I go to bed later I get up later. On the rare occasions that I go to bed substantially later (e.g. attending a concert) and so can't allow eight hours, I still cope well because I'm already well rested. Ditto for when my young son wakes in the night - previously this would have killed me.

I know this isn't viable for everyone but my employer allows flexible working hours.