Updated: Dec 16, 2020
The sleep-wake cycle is regulated by two important biological mechanisms. Sleep Drive or Process S is a pressure or propensity to fall asleep: the longer we stay awake, the more pressure we develop in our body to sleep. In Process S a drowsy substance, called adenosine, builds up in our blood during the day to gradually increase the sleep pressure as night arrives. The Circadian clock or process C is our internal body clock. We have powerful internal biological clocks that regulate our sleep patterns, digestion, core body temperature, and hormonal secretion. In process C, melatonin (sleep hormone) builds up in our body during the early evening and the first 2 hours of our sleep period. These two processes need to work together to have a balanced sleep-wake cycle.
Looking at the diagram below, we see sleep pressure (Process S) typically increases throughout the day ( the yellow arrows). However, it is moderated by the wake propensity generated through circadian drive ( the blue arrows) to keeps us awake until night time. At night the circadian clock begins producing melatonin. When this happens, the urge to sleep increases and the "sleep gate" opens. As the night goes by, the sleep pressure rapidly drops off but the sleep-promoting part of the circadian rhythm system takes over to keep us asleep for the rest of the night. By the early morning, melatonin secretion stops and the circadian alerting system begins and triggers the body to wake up ( blue arrows starts getting bigger and bigger).
Regular daylight exposure, regular meal times and exercise and social interaction regulate and set our circadian clock. Extensive light exposure during the night including lights from phones and computers may delay or decrease the amount of sleep hormone production.
“Sleeping in”, caffeine and naps weaken the sleep drive, making it harder to either fall asleep or stay asleep. Caffeine blocks the actions of adenosine and weakens the sleep pressure. It can take 6–8 hours for the effects of caffeine to wear off completely. Thus, drinking a cup of coffee in the early evening or late afternoon may prevent us from falling asleep at night.
The circadian rhythm works to keep us alert throughout the day despite the increase in sleep drive that builds up through the day. That's why for many shift workers, it is difficult to sleep during the day even though they haven't slept in the night before and have a strong sleep pressure.
There are some normal dips and high points of alertness during the day. Many of us feel most alert in the midmorning and early evening. For example, think about some toddlers who missed their naps and are cranky through the afternoon but all of a sudden they become very energetic and alert in the early evening. We also have a predictable drowsy time at midafternoon and like to have a post lunch nap.
Look at the graph one more time, we have the lowest alertness in early morning ( between 3-6 am). Try your best to avoid driving or any other actions that need full concentration and alertness during these hours because the biological clock is not sending out an alerting signal.
The diagram originally adapted from Edgar DM et.al 1993
The two process model was first introduced by the Swiss sleep researcher Alexander Borbély in the early 1980s
A short video about two process model of sleep :