Researchers recruited 120 undergraduate volunteers and assigned each by lottery to one of three groups for five consecutive nights. The eight-hour group was given a normal night's sleep, the six-hour group was woken two hours early, and the four-hour group four hours early. Participants slept in the laboratory so that sleep time could be verified rather than reported.

Each morning participants completed a task in which a light appeared at an unpredictable interval and they pressed a key as quickly as they could. The researchers recorded mean reaction time in milliseconds and the number of lapses, defined in advance as any trial with no response within half a second. Participants also rated their own sleepiness on a nine-point scale.

The table reports performance on the fifth morning. The researchers noted that the volunteers were students who answered a campus advertisement, and that all testing took place between seven and nine in the morning.

Performance on the fifth morning, by group

Group	n	Mean reaction time (ms)	Lapses	Self-rated sleepiness
8 hours	40	248	1.2	3.1
6 hours	40	297	4.8	5.0
4 hours	40	386	11.6	5.4

Lapses are trials with no response within 500 ms. Sleepiness is self-rated from 1 (fully alert) to 9 (fighting sleep).

Ten real questions.Every wrong answer explained.

Ten real questions.
Every wrong answer explained.