Many faces of sleep regulation: beyond the time of day and prior wake time

Abstract

The two-process model of sleep regulation posits two main processes regulating sleep: the circadian process controlled by the circadian clock and the homeostatic process that depends on the history of sleep and wakefulness. The model has provided a dominant conceptual framework for sleep research since its publication ~ 40 years ago. The time of day and prior wake time are the primary factors affecting the circadian and homeostatic processes, respectively. However, it is critical to consider other factors influencing sleep. Since sleep is incompatible with other behaviors, it is affected by the need for essential behaviors such as eating, foraging, mating, caring for offspring, and avoiding predators. Sleep is also affected by sensory inputs, sickness, increased need for memory consolidation after learning, and other factors. Here, we review multiple factors influencing sleep and discuss recent insights into the mechanisms balancing competing needs.

Keywords: circadian clock; motivation; sleep; sleep homeostasis; two-process model.

Figure 1. Multiple factors influencing sleep.

In addition to the time of day and prior wake time, sleep is strongly influenced by other factors, a subset of which is depicted. Blue arrows indicate sleep promotion, and red lines indicate sleep suppression. The separate arrows and lines do not represent independent processes. The various factors may influence sleep in part through their effects on the circadian and homeostatic processes.

Figure 2. Nutrition modulates the balance between competing needs.

The nutritional status shifts the balance between sleep and other needs. Top: Under good nutritional conditions, male flies forgo their nighttime sleep to engage in reproductive behaviors. Lack of dietary protein, needed for offspring survival, causes males to reduce their nighttime courtship in favor of sleep [93]. Bottom: Garden warblers in good nutritional conditions choose to sleep in an untucked position, which allows for higher alertness and predatory avoidance. Poorly nourished warblers sleep with their heads tucked under the feathers, which favors energy conservancy over predator avoidance [249].