The forbidden zone of sleep (FZS) is defined as a period of time of natural wakefulness right before the onset of nocturnal sleep. During this period, the body has a high tendency to resist sleep and it is usually impossible to fall asleep. Under normal sleep conditions (based on normal nocturnal monophasic sleep and no sleep deprivation), FZS hours vary from 19:00 to 21:00 or 22:001. It is worth noting that the regular FZS hours are not set in stone and have the potential to vary between individuals depending on lifestyle. Knowing that this zone exists and how to schedule sleeps around this zone is a perk for polyphasic sleeping. Questions to be answered include, but are not limited to, whether polyphasic sleeping applies the same principles of FZS, are the FZS hours different on polyphasic schedules and if these hours can be shifted, how can they be different? Why do some certain polyphasic schedules not seem to follow the regular hours of FZS? There are more questions than answers, so this blog will attempt to decipher how the FZS works under different sleep conditions (e.g, sleep deprivation’s involvement or not), and whether the standard polyphasic schedules enlisted in this community guideline follow the rules of FZS or not. This blog post will break down FZS hours under normal nocturnal sleep at regular hours (normal lifestyles) and irregular hours (e.g, shift work).
The role of thyrotropin hormone & sleep deprivation
It has been recently hypothesized that thyrotropin (thyroid-stimulating hormone) surge during the hours of FZS acts as a survival enhancer for human beings, from an evolutionary perspective2. From the circadian rhythm standpoint, it is observed that thyrotropin level is low during noon or early afternoon where sleeping is facilitated, and the hormone level raises during FZS, effectively preventing sleep2. In terms of survival mechanisms, one proposal is that humans are vulnerable to different threats when the night falls (stemming from ancient time), so a period of preparation for safety before night sleep is prioritized. These hours coincide with the FZS and over time humans have become accustomed to staying awake during these evening hours2.
Thyrotropin was declared as a circadian pacemaker that gives signals to the body of sleep hours. The pattern of thyrotropin levels is specifically as follows: Sharp increase in the evening and peak around the time of sleep onset, and decrease during sleep. During sleep deprivation scenarios (e.g, most polyphasic sleep adaptations), thyroid hormone is said to sharply increase2,3. This is a survival mechanism against sleep deprivation. In hypersomniac individuals, thyroid level is low, causing constant sleepiness during the day; on the other hand, insomniacs or sleep deprived individuals have elevated levels of thyroid in the system.
During sleep deprivation periods in polyphasic sleep adaptations, it can be inferred that partial sleep deprivation (not to be confused with total sleep deprivation, which denotes no sleep) causes thyroid levels to increase. As sleep deprivation persists, at one point thyroid-stimulating hormone (TSH) levels decline because of negative feedback by thyroid on TSH release3. More sleep deprivation is also associated with stress and theoretically forces the sympathetic nervous system to release more TSH. Once the sleeper rests, the thyroid hormone level declines.
One hypothesis to be concerned about is that it is unknown whether continuous failed polyphasic adaptations can lead to the chaotic regulation of thyroid hormone or instinctively lower thyroid release in the future when an adaptation is kick-started again, making both falling asleep and oversleeping very easy and occur more often than before. Similar to the Oversleeping Syndrome addressed in the community, when the body is used to oversleeping, and in the midst of sleep deprivation, sleepers can sleep through all alarms and even stronger external stimuli that they normally would be able to respond to.
Sleep in the forbidden zone
One would ponder if it is completely impossible to sleep in the FZS. People have slept with irregular patterns before, including hours much earlier than the regular night sleep hours, which coincide with the forbidden zone. The answer to this question is, “no”. Under sleep deprivation conditions, homeostatic pressure can take over FZS, facilitating sleep at these hours4. It is also worth noting that even though 21:00 is borderline with FZS, it is possible and reasonable to start sleeping at this evening hour (if it fits your lifestyle) because natural melatonin release starts at 21:006.
However, under conditions with no sleep deprivation, FZS naps become very light and easy to wake from, giving negligible sleep inertia but also less restorative values than earlier daytime naps1,5. It is also observed that naps in this zone generally contain less vital sleep stages and more light sleep, hence the easier wake compared to earlier naps in the day. Age is also another factor to be considered. In one study, it was hinted that adolescents may naturally have longer FZS duration than adults under no sleep deprivation7. This interpretation is made based on the results that adolescents struggled to fall asleep 2h before dim-light melatonin onset more than adults, which is a potential contributor to FZS duration. This can be something to note when scheduling different polyphasic patterns.
How the forbidden zone is applied in different polyphasic schedules
Here, a theoretical visualization of FZS in different schedule groups will be presented, under normal scheduling conditions (the focus is on nocturnal sleep with added naps). Note that these illustrations of FZS can vary slightly from person to person (e.g, FZS duration).
Out of the four known nap-only schedules, SPAMAYL is the only one that appears to defy the normal FZS rules. The Uberman pattern can be scheduled around FZS hours by having 6 equidistant naps placed at different hours. Regarding SPAMAYL, it is unknown whether chaotic napping can alter the timing of FZS, even though scheduling can be tweaked to avoid the normal FZS hours. While it is evident that some nap-only variants do not obey the traditional FZS hours, this is negligible because they require intense sleep deprivation to raise the homeostatic pressure to very high levels to be able to nap in FZS, overriding the effects of FZS.
Deterministic behavior of the forbidden zone
Those who do not sleep in the normal graveyard hours (midnight to 08:00) probably set different FZS hours for themselves. It is predicted that those who have a shifted circadian rhythm, whether forward (delayed phase) or backward (advanced phase) will set different FZS for themselves. A lot of polyphasic sleepers have adapted to different sleep times in the day, whether their main core sleep is at the same or different hours than that of their normal monophasic routine. As a result, the regular FZS hours may no longer apply in these scenarios and theoretically become subjected to changes in sleep times.
A study on shift workers showed that different phases of the circadian rhythm determines how an FZS is formed, which is marked by melatonin onset8. It was demonstrated that morning shift workers’ preferred habitual sleep time dictates sleep duration and there is a FZS involved. For example, one important observation from the study is that those who have to wake up earlier in the morning for the shift work still sleep at their normal hours at night, which results in a reduction of night sleep duration- the reason for this sleep reduction is explained by their inability to sleep at earlier hours (not because of social commitments to stay awake in the evening hours). Another study supported the idea that FZS hours depend on habitual sleep times in each individual9. For example, those who sleep at 04:00 consistently may see their FZS start a couple hours leading up to 04:00, rather than the regular 19:00-21:00 range as postulated.
Additionally, it is important to consider sparing at least some waking hours apart from each sleep, especially in polyphasic schedules with a shifted circadian phase (whether forward or backward). A well-defined, consistent bedtime on a daily basis will help shape FZS hours over time, regardless of whether the bedtime coincides with the timing of the nocturnal monophasic core sleep. Thus, when there is a consistent phase shift in bedtime, FZS hours are affected and shifted accordingly.
Is it possible to erase the forbidden zone on a polyphasic schedule?
The short answer is likely no. It is likely that when the adaptation process is completed, sleepers will become familiar with the new sleeping hours and their FZS may or may not be different from that on their monophasic schedule. It is worth noting that no research up to date has shown that FZS can vary on a daily basis, but rather appears to be set in stone. Completely random sleep patterns can weaken circadian cues, and eventually lead to a circadian desynchronization with unpredictable sleep and wake times (check Random sleep for more information). The concepts of day and night may no longer be defined (similar to jet lag effects). Thus, having a stable circadian rhythm with a clear sleep-wake boundary is better suited with consistency of sleep times. It is also safer and healthier to practice sleep hygiene to keep FZS consistent on a daily basis.
FZS is a very intriguing yet complicated concept. There are still a lot of unknowns in this area of sleep, as well as its interaction with certain hormones in the body. The most important factors that manipulate FZS are personal bedtime and lifestyles. Even though there are certainly some mechanics around scheduling a polyphasic sleep pattern, there are still more questions regarding their true long-term viability in mainstream science. Generally, it would be best to follow a stable sleep pattern with certain consistency to maintain a well-defined FZS and balanced hormones. However, the human body can adapt to different living conditions, and mainstream knowledge of nocturnal monophasic sleeping cannot exclusively be used to explain polyphasic sleeping mechanics1.
Main author: GeneralNguyen
Page last updated: 12 May 2020
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- José, et al. “The.” Sleep Science, vol. 4, no. 3, 2011, pp. 105–109, www.sleepscience.org.br/details/74/en-US/the–forbidden-zone-for-sleep–might-be-caused-by-the-evening-thyrotropin-surge-and-its-biological-purpose-is-to-enhance-survival–a-hypothesis. Accessed 20 Feb. 2020.
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