Sleep homeostasis and circadian rhythms regulate when we feel alert and when we feel tired and fatigued. These two processes are at the core of many models addressing the regulation of fatigue and performance (Achermann, 2004). There are other factors that influence our level of fatigue (e.g. quality of sleep the previous night, level of activity, levels of interest in the task being performed, lighting).
Nevertheless, sleep homeostasis and circadian rhythms are the strongest factors influencing our level of alertness.
Sleep homeostasis is the amount of time awake since the last period of sleep. obviously, workers are going to feel more tired if they have not slept in the last 16 hours than if they had just woken up from a nap an hour ago. A related factor is the cumulative effects of suboptimal periods of sleep over extended periods of time. Two nights with four hours of sleep each produce ‘pathological’ levels of sleepiness. Periods of recovery sleep are essential to allow a return to ‘normal’ levels of alertness. After seven days of only five hours of sleep each night, one full night of sleep followed by a day-time nap returned measures of sleepiness and fatigue to pre-restricted levels (Carsakadon and Dement, 1981).