Shapiro - ultramarathon runners showed longer sleep after intense physical activity and increased proportion in deep sleep, suggesting it physically recovers body
Haider - EEG of 10 patients recovering from drug overdose showed more time spent in REM, suggesting it is for mental recovery
Therefore, this demonstrates evidence of stages of sleep
Individual differences
Significant variation between people
Tucker et al. - large differences between patients in terms of duration of each sleep stage, particularly 3 and 4
Concluded that these stages are likely to be biologically determined
Therefore, difficult to describe 'normal sleep' in any meaningful way
