Three past normal ?

"When the rats were gently nudged to stay awake three or six hours past their normal sleep times"?

I cannot get the meaning of "three or six hours past their normal sleep times" clearly. Supposed the rats' normal sleep time is 8 hours, what is 3 or 6 hours past the 8 hours? Gosh, I am so confused.


Context:

The authors measured levels of adenosine triphosphate (ATP), the energy currency of cells, in rats. They found that ATP levels in four key brain regions normally active during wakefulness increased when the rats were in non-REM sleep, but were accompanied by an overall decrease in brain activity. When the animals were awake, ATP levels were steady. When the rats were gently nudged to stay awake three or six hours past their normal sleep times, there was no increase in ATP.

http://www.sciencedaily.com/releases/2010/06/100629170924.htm