How are people able to wake themselves up after a pre-specified amount of time?

Question

Auto-suggestion seems to work. Several people I know seem to be able to tell themselves "I'll wake up after 'N' hours/minutes", and do so within a minute or two of that duration.

I thought for a moment that it might be that the duration defined is typical to the person, but even defining a weird figure (e.g., 23 minutes), it appears to work just as well as that person's regular duration of 15 minutes.

Personally it works for me sometimes (usually when I'm under pressure), other times it does not.

How does the brain/body know to power-up after N minutes?

Answer 1

There is quite a lot of research on self-awakening (see this search on Google Scholar for self awakening). Hopefully someone else more familiar with this literature can add a more authoritative answer about the mechanisms of self-awakening. In the interem I briefly extract some relevant points from Ikeda and Hashi (2012). The study does not directly address biological mechanisms. However, understanding the reliability of self-awakening and the correlates of the ability to self-awaken is presumably relevant to forming such an understanding.

First they summarise some existing research:

Both the accuracy and success rate of self-awakening have been experimentally examined. More than half of the people who have the ability to self-awaken suc- cessfully awakened within 30 minutes of the predetermined time. For example, seven participants succeeded on nine of 14 days (64%) in a sleep laboratory, and 15 participants succeeded on 35 of 44 nights (80%) at their homes. Survey studies indicate that many people habitually self-awaken in daily life; for example, 52% of 269 adults (aged 21−84 years) and 10.3% of 643 university students6 reported habitu- ally self-awakening. People who have a habit of self-awakening in the morning have regular sleep/wake schedules, tended to have a morningness chronotypology, awakened comfortably in the morning, and had less daytime dozing.

In their own study they found:

The present study investigated self-awakening, both habitual and inconsistent, compared to awakening by external means in relation to sleep/wake schedules for five consecutive years in 362 students (starting at mean age 15.1 ± 0.3 years). Students who self-awakened consistently for five consecutive years (5% of all students) went to bed earlier than those who inconsistently self-awakened (mixed group, 40%) or consistently used forced awakening by external means (56%). Awakening during sleep was more frequent and sleep was lighter in the consistently self-awakened group than in the mixed and consistently forced-awakened groups. However, daytime dozing was less frequent and comfort immediately after awakening was greater for the consistently self-awakened group than for the mixed and consistently forced-awakened groups.

Reference

• Ikeda, H., & Hayashi, M. (2012). Longitudinal study of self-awakening and sleep/wake habits in adolescents. Nature, 4, 103-109. PDF
• Bell CR. Awakening from sleep at a pre-set time. Percept Mot Skills. 1980;50(2):503–508.
• Lavie P, Oksenberg A, Zomer J. “It’s time, you must wake up now.” Percept Mot Skills. 1979;49(2):447–450.
• MoorcroftWH,KayserKH,GriggsAJ.Subjectiveandobjectiveconfir- mation of the ability to self-awaken at a self-predetermined time without using external means. Sleep. 1997;20(1):40–45.
• ZepelinH.REMsleepandthetimingofself-awakenings.BullPsychon Soc. 1986;24(4):254–256.
• Zung WW, Wilson WP. Time estimation during sleep. Biol Psychiatry. 1971;3(2):159–164.
• Matsuura N, Hayashi M, Hori T. Comparison of sleep/wake habits of university students with or without a habit of self-awakening. Psychiatry Clin Neurosci. 2002;56(3):223–224.

Answer 2

The short answer is that this is unknown. However, there are a few potential leads.

One hint is that people are able to estimate the passage of time fairly reliably, and this ability seems to persist during sleep (Ukraintseva, Liaukovich, & Shilov, 2021; Aritake-Okada et al, 2010):

The notion that [time estimation ability] operates in the brain even during sleep is supported by, for example, “self-awakening” ... and “anticipated sleep termination” ... This research found that ... subjective elapsed time during sleep was very close to actual sleep time ...

Furthermore, the ability to estimate time during sleep can vary substantially. Moorcroftl & Breitenstein (2000) review current speculation:

Source of self-awakening ability

In addition to imputed read-out of an internal biological clock, speculation about the source of this awareness of time during sleep has also included light sleep with frequent awakenings and periodic REM sleep. ... Bell has proposed that the ability to self-awaken consists of two components: the ability to induce more frequent awakenings coupled with the ability to use these awakenings to mark the passage of time.

Accordingly, successful self-awakening is associated with factors that increase "arousal" (alertness) during sleep such as lighter sleep, shallower sleep phases (ie, REM), lower waking threshold, increased brain activity, reduced sleep inertia (grogginess), and higher sleep quality (Ikeda & Hayashi, 2012; Matsuura & Hayashi, 2009; Malloggi et al, 2022). This presumably increases both time estimation accuracy and ease of waking. Hayashi, Matsuura, & Ikeda (2010) summarize current findings with this schematic:

Additionally, one study suggests that successful self-awakening is associated with increased brain activity particularly in the right prefrontal cortex, which incidentally, is involved in time estimation (Aritake et al, 2012).