The two-process model of sleep regulation is based on the interaction of our two bodily processes – sleep-wake homeostasis (S) and the circadian process (C) – working simultaneously and dictating our sleep length, quality, and it’s timing.
While falling asleep can be the easiest thing in the world, especially after a long, hard day at work, there are a lot of factors influencing our sleep-wake time other than our conscious efforts of drinking coffee or setting the alarm for 7 A.M. Falling into slumber is an instinctive element of our daily lives. So much so, that we seldom give it a second thought until bad sleeping hygiene noticeably takes a toll on our daytime activities and wellbeing. The thing is, sleep is a major factor when it comes to our health, impacting most, if not all of our bodily processes, and has the potential to wreak havoc when neglected. A lot of complications with the immune system, metabolism, even mental health, can be affected or caused by a particularly bad sleeping spell, but unless it’s very obvious, it regularly gets addressed as the last resort.
It isn’t helped by the fact that sleep research, until the end of the 19th century, didn’t have many methods available for experimenting and extending the sparse knowledge we had at the time. Only with the discovery of EEG (electroencephalography) in the 20th century was this ice broken, and many other revelations followed, setting the ground for sleep science to develop. That is why this area is still very much considered new and has yet to be unlocked.
The two-process model of sleep regulation is an important sleep research posit explaining what goes on in our bodies during the 24-hour time frame, making us feel energized and awake at some points, drained and fatigued at others. It’s based on the interaction of our two bodily processes – sleep-wake homeostasis (S) and the circadian process (C) – working simultaneously and dictating our sleep length, quality, and it’s timing. Before explaining how exactly these two combine, let’s take a look at what each of them is.
Homeostatic processes throughout the body are basically in charge of small-range maintenance, each in their own respective area. One homeostatic process, for example, is in charge of thermoregulation; another one monitors the salinity level in our blood, etc. The sleep-wake homeostasis, or process S, is a sleep-dependant process, in charge of generating a sleep drive in your brain, making you progressively more and more tired as the day passes by to ensure you fall asleep after enough time has passed. When you do fall asleep, the process is set off and gradually declines until you wake up. This sleep pressure seems to be connected to the NREM (non-rapid eye movement) phase of sleep, characterized by slow wave activity (SWA) in the brain. Because of this, SWA is perceived as one of the markers of process S in the brain. Another sign of sleep pressure or hypnogenic build-up is a molecule called Adenosine. Sleep drive has been shown only to decrease while in the NREM stage of sleep, other stages do not offset it entirely.
Certain timing of feeding and sleeping patterns is present in all animals, including humans. The circadian process, or process C, is simply put our internal, biological clock. It is a sleep-independent process that controls the body’s alertness levels and keeps check of our feeding time, brain wave activity, heart rate, oxidative stress, immune responses, hormone production, etc. It is situated in the small section of our brain’s hypothalamus known as suprachiasmatic nucleus, right above the optic chiasm, which consists of over 20000 neurons that respond to light. Process C looks over our 24-hour schedule; synchronizes our sleep rhythm with external cues, mainly light levels, to make sure we sleep when it’s dark and wake up in the morning, while also regulating other internal processes accordingly. It intervenes if the sleep drive gets too high during the daytime – without it, our sleep patterns would be completely random, unrelated to external stimuli, and more prone to polyphasic sleep. Circadian rhythm is assumed to be controlled by a circadian oscillator, and it’s markers in the body are melatonin secretion, core body temperature and plasma level of cortisol.
These two processes constantly coexist and override each other during different times of the day. A normal day (meaning, after a good night’s sleep, and without activities that require attentiveness late at night) would look something like this:
You wake up and slowly pick up your daily pace. Sometime after lunch, your energy level drops a bit. Maybe you even take a nap, which freshens you up just enough so you can push through the rest of the work, wrap up your to-do list and call it a day. Later, comfortable and finally in bed, sleepiness takes over, and you quickly doze off.
This example shows the two-process model working regularly. Both of these processes have a peak and an ebb during the 24-hour cycle. The morning, after we shrug off sleep inertia, is typically when our mental capability levels at their highest. Process S slowly starts picking up and builds pressure as the day goes by. Circadian rhythm maintains this pressure and keeps it in check, so as to help prolong our time awake until nightfall when it enables melatonin production and induces sleep. Plainly explained, Process S peaks right before you fall asleep, and is at its lowest right before you wake up. Process C is at its ebb when you fall asleep, and peaks sometime midday.
What this essentially means by extent is that process S and process C might gang up on you when you are trying to stay awake after not sleeping for an extended period. Such a situation isn’t uncommon when working at a shift job. Case in point: After being awake the whole day, a surgeon, the only one available at the time, has to operate at 5 A.M. The sleep drive in his brain is over the roof, and the circadian rhythm isn’t making it any easier to keep his eyes open, insisting that it is nighttime and he should be sleeping. This sleepy cloud can seriously compromise our abilities and put us at risk of injuries and accidents, not to mention cause sleep deprivation issues if it happens on a more regular basis.
However, there are also times where the two processes work against each other, usually prompted by a sudden change in our surroundings or a stretch of our wakeful state beyond the regular sleep schedule.
The perfect example is jet-lag; changing time-zones so suddenly leaves your body confused for up to a few weeks. As a result, as tired as you may feel, your circadian rhythm refuses to let you rest at night because according to recent experience, it’s not supposed to be night. It hasn’t yet successfully tuned your internal clock to match the day-night rhythm of the zone.
Another testimony to these two processes clashing occasionally occurs when you come home in the early morning after pulling an all-nighter and find it impossible to fall asleep. Once again, it’s process C, letting you know that you missed your shot because the night is over. Although it can create quite a struggle sometimes, this strict internal clock sometimes covers for you, helping you stay awake during the day even after a slim sleeping session the previous night, or fall asleep at night even though you weren’t feeling very tired.
It’s important to remember that process S and process C function together to balance your sleep habits and aren’t independent, which is perfectly displayed in the fact that the clock genes (which control circadian rhythm) have a role in the homeostatic process as well. They are connected, and both required in order to provide fulfilling rest. If the circadian process becomes too strong, it can prevent you from catching up on sleep after being deprived. In the other direction, sleep deprivation can also weaken the circadian process. This Two-Process Model is helpful to navigate our sleep-wake cycle and predict daytime vigilance levels, but a lot of other factors also come to play when it comes to our general sleep and well being.
Although this model is very helpful in understanding what happens in our brain in different stages of the day, you have probably realized by now that you cannot rely on your body to be impeccable in every situation and fix problems it didn’t cause. There’s almost always something else interfering with the system externally and making things more complicated. Some of these factors have a sedative effect on you, some offset sleep, and most of them you have either heard of or are guilty of yourself.
– Irregular sleeping schedule. Maybe you switch shifts often; maybe just willingly postpone sleep to watch a show, then take long naps to try and make up for it. When this scattered behavior happens often, it can take a toll on your body. Your circadian rhythm desperately tries to make sense of what you are doing, causing you to feel tired pretty much all the time and not letting you sleep when you want. To avoid this effect, try sticking to a plan of going to bed at the same time every night, and waking up the same every morning. To fit this in your weekly schedule, find the earliest time you must wake up, maybe 7 A.M. on Wednesday for that 8 A.M. class, and then wake up at that time every other day as well. That way, your circadian clock will work to your advantage, plus you will guarantee not feeling lost in class on Wednesday.
Tanya is a professional writer and editor with a B.A. in English from the University of Chicago. Tanya has been fighting insomnia for most of her adult life, and she knows firsthand how vital a good night’s rest can be for people with sleep problems.