Lack of sleep has been blamed for everything from poor driving to memory problems. Now, researchers think they’ve started to pinpoint why: week-long sleep studies of people resting for 10 hours a night vs. six hours showed significant changes in gene activity.

When sleep was restricted to under six hours a night, researchers noted that 444 genes showed suppressed activity, and 267 genes showed more activity. Those genes control everything from the body’s immune system to its reaction to stress.