The FAA.
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http://www.faa.gov/regulations_polic...-FinalRule.pdf

Excerpts:

Fatigue is characterized by a general lack of alertness and degradation in mental and physical performance. Fatigue manifests in the aviation context not only when pilots fall asleep in the cockpit in flight, but perhaps more importantly, when they are insufficiently alert during take-off and landing. Reported fatigue-related events have included procedural errors, unstable approaches, lining up with the wrong runway, and landing without clearances.

Common symptoms of fatigue include:
• Measurable reduction in speed and accuracy of performance, Lapses of attention and vigilance, Delayed reactions,
• Impaired logical reasoning and decision-making, including a reduced ability to assess risk or appreciate consequences of actions,
• Reduced situational awareness, and
• Low motivation to perform optional activities.

A variety of factors contribute to whether an individual experiences fatigue as well as the severity of that fatigue. The major factors affecting fatigue include:
• Time of day. Fatigue is, in part, a function of circadian rhythms. All other factors being equal, fatigue is most likely, and, when present, most severe, between the hours of 2:00 a.m. and 6:00 a.m.

• Amount of recent sleep. If a person has had significantly less than 8 hours of sleep in the past 24 hours, he or she is more likely to be fatigued.

Scientific research and experimentation have consistently demonstrated that adequate sleep sustains performance. For most people, 8 hours of sleep in each 24-hour period sustains performance indefinitely. Sleep opportunities during the WOCL are preferable because sleep that occurs during the WOCL provides the most recuperative value. Within limits, shortened periods of nighttime sleep may be nearly as beneficial as a consolidated sleep period when augmented by additional sleep periods, such as naps before evening departures, during flights with augmented flightcrews, and during layovers. Sleep should not be fragmented with interruptions.

Sleep science has settled on the following points: The most effective fatigue mitigation is sleep; an average individual needs to have an 8-hour sleep opportunity to be restored; and daytime sleep is less restorative than nighttime sleep. For most people, 8 hours of sleep in each 24 hours sustains performance indefinitely. There is a continuous decrease in performance as sleep is lost.
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There's a lot more, but it's a 300+ page document and I'm not going to spend a lot more time searching. The above didn't take me very long.