The 2-process model of sleep regulation
What is it that causes us fall asleep and wake up? Have you ever tried catching some shut eye during a free period in the day only to find that it is not so easy to fall asleep whenever you want to? Or why is it that even after spending the entire day on the go, we sometimes feel unusually alert in the evening?
Sleep is thought to be governed by 2 processes: (1) sleep-wake homeostasis and (2) the circadian process.
Sleep pressure builds up with sustained wakefulness and dissipates with sleep. One factor contributing to the build-up of sleep pressure, causing us to feel sleepy is the increase in adenosine, a molecule that is produced as the brain uses energy. Caffeine reduces sleepiness by blocking adenosine receptors in the brain. Neural networks tend to go offline after sustained wakefulness. This is another reason performance degrades with sleep loss.
The circadian process
An internal clock within the SCN (a part of the brain) orchestrates sleep-wake behaviour around a 24 hr cycle - hence the name 'circadian' which means 'about a day'. An alerting signal builds up shortly before waking, reaching its peak in around noon. Sometime in the middle of the afternoon, the alerting signal dips, causing a bout of afternoon sleepiness (yes, this happens no matter what you’ve eaten for lunch!). However, the alerting signal quickly picks back up and continues to rise into the early evening. Just before bedtime, the alerting signals begin to fall, and the signals that make us sleepy start to increase again.
These two processes work together. We feel energized in the late morning because sleep pressure is at its minimum and circadian alerting signals are high (if you feel sleepy even in the mid-morning, you are very likely not getting enough sleep at night). As night time approaches, the homeostatic signals driving sleep keep increasing, and circadian alerting signals begin to fall. This combination triggers a strong desire to sleep. At the time of waking, sleep pressure has been relieved, but the circadian alerting signal stays low accounting for why some still feel sleepy in the early morning.
Variations in sleep rhythms
Persons show differences in preferred bedtime and waketimes that are independent of work schedules. Persons who are “morning types” go to bed early and wake up early. They function best in the earlier part of the day. Conversely “evening types” prefer to sleep later and get up later. Although these preferred rhythms are biologically governed, standard school hours and work schedules are biased toward early start-times. This situation is punishing for individuals with a “delayed” schedule i.e. evening types.
In adolescence, biological changes delay the time that one feels sleepy. This sleep phase delay lasts till early adulthood. As a result, teens only start to get sleepy later compared to children or older adults. Moreover, ever-increasing school demands drive students to make the conscious choice to extend bedtimes even later, resulting in unnecessarily shortened sleep durations for many teenagers.
Early school start times are an added burden to a teen experiencing a sleep phase delay – forcing a teen to wake earlier than what their circadian rhythm would dictate is akin to a cold engine taking a long time to warm up before it can function as it is supposed to. This situation, coupled with high levels of daytime sleepiness as a result of insufficient sleep the night before, lead one to become lethargic and grumpy in the day – not at all an optimal state for learning or exercising creativity in the classroom.
A lot to take in? Sleep on it!
The idea that “sleep is for the weak” may have come about because sleep is thought to be a cognitively barren unconscious state that hinders productive thought and study. This is not at all true. In contrary, there are active processes that go on during sleep.
Among the various functions of sleep, it’s role in learning and memory is particularly important. Sleep promotes the consolidation of memory; a process whereby new and initially fragile memories are transformed into more stable representations that are integrated into a network of long-term memories. This is achieved by the re-processing of memories in a sleep-specific neurochemical environment of electrophysiological activity. Notably, if your sleep is cut short, fragmented or impoverished due to stress and anxiety, these sleep processes are interrupted and cannot achieve its memory enhancing functions.
Sleep, compared to an equivalent period of wake, improves memory for object locations, short stories, wordlists, and memory for intentions that have to be executed at a later time. A recent study from our lab found that for Singaporean students who napped after learning versus those who spent the same period of time further cramming the material, memory retention was not worse, but was in fact equivalent, and more durable. Practice makes perfect, but only if you allow sleep the opportunity to consolidate your practice efforts.
These findings are of great importance in educational settings and should shape how students learn and study.
The challenge of bring about real change requires a combined effort from individuals, families, educators and leaders in the community and political spheres.
How we measure sleep
There are many ways scientists assess sleep. In our lab, we use subjective measures such as questionnaires and sleep diaries, as well as objective measures such as actigraphy and polysomnography.
An actiwatch is a wrist-worn monitoring device frequently used in research settings to objectively evaluate sleep by recording and allowing comparisons of levels of light intensity and physical activity across a 24-h period.
Polysomnography (PSG) is deemed the ‘gold-standard’ to measure sleep objectively. This is a technique whereby electrodes are placed on the scalp and face to monitor brain activity, eye movements, and muscle tone during sleep. Sometimes, a nasal airflow sensor and pulse oximeter may be included to monitor airflow and the level of oxygen in the blood. For our NFS studies, we use a portable PSG device by Somnomedics so that participants can move around freely even with it attached to the head. The use of this device on a scale such as ours is unprecedented.