The research: <a href="http://www.bmj.com/cgi/content/full/310/6979/565" rel="nofollow">http://www.bmj.com/cgi/content/full/310/6979/565</a><p>There is no such thing as a sleep debt in the sense that it accumulates over a week.  You could go days without sleep and only need 8 hours to get back to perfect.  However, in the course of a 24 hour period.  If you only have 4 hours of sleep you obviously have a sleep debt of 3-4 hours and need to take a power nap or something