Sleeping in on Saturday after a few weeks of too little shuteye may feel refreshing, but it can give a false sense of security.

New research shows chronic sleep loss can't be cured that easily. Scientists teased apart the effects of short- and long-term sleep loss — and found that the chronically sleep-deprived may function normally soon after waking up, but experience steadily slower reaction times as the day wears on, even if they had tried to catch up the previous night.