Sleep Disorders

The two-process model of sleep regulation

The dominant model for sleep regulation today is the two-process model. These two processes – the homeostatic process and the circadian process – sometimes work together and sometimes against each other, and the situation and wishes of the person can often override the process and keep us awake when we should be sleeping.

Homeostatic just means the body tries to maintain itself in a narrow range. Body temperature is regulated in a homeostatic process – the body stays with a few degrees. So are the pH and salinity of bodily fluids. This is a little simplistic, but sleep homeostasis can be modeled by assuming there is a quantitative need for sleep that builds while a person is awake and declines during sleep. The homeostatic pressure to sleep depends not only on how long you are awake but on how active you are while awake. What constitutes this need for sleep in a biochemical sense is still a subject for research. Discovery of orexin and unraveling of some of the neurotransmitter soup in recent decades have helped paint part of the picture but even the top neuroscientists confess they don't know.

The circadian process helps keep the homeostatic process in line. It helps us sleep through the night and not drop off during the day. Without the circadian process we would sleep intermittently in a manner more like polyphasic sleep. Physiologists who use this model often call the homeostatic process "Process S" and the circadian one "Process C."

The two-process model gets some vindication when you look at the stages of sleep in a typical night. Earlier in the night the periods of deep sleep (stages 3 and 4) are longer and more frequent than they are later in the night.

Army researchers have recently been able to fit a biomathematical model of sleep-related performance impairment using a two-phase process.

When sleep quality declines as people get older, the cause can be a slip in either or both of the processes.