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Picture of the brain under flourescence with the suprachiasmatic nucleus highlighted in red Circadian Time of Day

Humans are diurnal creatures.  We each have an internal circadian rhythm that drives us to be awake during the day and asleep at night.  This rhythm is generated by suprachiasmatic nuclei situated in the hypothalamus (the area highlighted in red in the figure of the brain to the right). As a result of this internally generated drive, it is very difficult to be at maximal performance in the middle of the night even when fully rested. 

 

The circadian rhythm controls much more than the drive for sleep and wake, which means many other systems are affected when one stays awake all night. 

The figures below show several biological outputs that are controlled by the circadian system. The plot to the left comes from a review of endocrine changes controlled by the SCN by Czeisler and Klerman (1999). The "Entrained" column shows the diurnal component to each output when a person regularly sleeps at night (W=wake, S=Sleep). The "Constant Routine" column shows how each output oscillates independent of sleep. In the constant routine condition, study subjects were kept awake overnight in dim lighting conditions. The differences in the rhythms illustrates whether an output is controlled by the circadian system.
   

Two plots showing the variation of hormones such as melatonin and cortisol with the circadian rhythm and short-term memory consolidation

For example, in this graph it is easy to see how the rhythm of melatonin is unchanged in both conditions demonstrating its circadian control, while the rhythm of growth hormone is blunted in the constant routine condition, demonstrating its sleep dependent control.

The adjacent figure shows the results of a study by Johnson and colleagues that demonstrated that the rhythm of short-term memory recall is under circadian control as are cognitive performance, subjective alertness and core body temperature (1992).

The results of each of these studies demonstrates that the circadian rhythm oscillates independent of sleep. This means that a large drop in the rhythms of cognitive performance and short-term memory will occur when one stays awake at night.

Hospitals must maintain 24-hour operations, but the effects of working at night can lead to serious consequences especially when compounded by continuous hours awake, chronic sleep deprivation and sleep inertia. Eliminating work at night is clearly not an option.  The best way to counter the effects of circadian time of day is to design schedules that take all of the other biological determinants of sleepiness into account.  For example, by reducing the number of continuous hours awake, such as with a split call design, the decrement in performance in the middle of the night becomes less severe. Teaching residents to implement sleep hygiene strategies will also help minimize the decrease in performance in the middle of the night.