Sleep

Here's the deal: you need 8 hours. The body's sleep "cycle" is controlled by two things

1) Homeostasis - your body wants to be sleeping 1/3 of the day (8 hours) (Theoretically, it doesn't matter when you get those 8 hours... you can sleep for an hour, be up for 2 hours, go to sleep for an hour, be up for 2, sleep for 1, etc. But that's not actually how it works because of the other control system

2) Circadian rhythms - your body had "alertness" impulses that are at their lowest during the hours of 10 pm - 9am. This is controlled mostly by input to the brain in the form of a hormone called melatonin and the external cue of light.

Basically, your eyes have rods and cones to see things, but they also have retinal gangion cells that receive light only for the purpose of telling your brain what time of day it is. Then your body and mind know when to be awake and when to sleep.

If you're like me and have trouble sleeping here's the take home message: get plenty of light during the daytime/productivity time of 7 or 8 am until about 9 or 10 pm (yes, hard to do while in a dark lecture hall for a good portion of the day). Then turn off the lights (and bright computer screens that your eyes mistake for daylight) and go to bed. Be consistent with this. Sweet dreams!