After watching The Martian, I had falsely assumed that a day on mars is significantly longer than on earth, but it is actually only 40 minutes longer.

However, the question for me still remained: Can humans acclimate to shorter/longer days?

A more detailed follow up: Could a human acclimate to a 16 hour day? Or in the opposite direction, can a human acclimate to a 32 hour day?

I'm looking for studies and experiments in proportionately adjusting the awake and sleep time of a human. This should hopefully include changing the length of day and night artificially, such that a human would sleep during dark periods.


Yes, it can.

The "master clock" of your body is the suprachiasmatic nucleus of the hypothalamus. It receives:

  • Neuronal input from the retina, through the retino-hypothalamic tract.
  • Hormonal input, as the suprachiasmatic nucleus expresses melatonin receptors. Melatonin is a hormone secreted by the pineal gland in response to the absence of light. The pineal gland itself does not contain light receptors, but it received information indirectly: the retino-hypothalamic tract sends the information from the retina to the suprachiasmatic nucleus. From there, the information is sent to the intermediolateral column of the spine cord, where there are synapses with pre-ganglionic neurons which also synapse with post-ganglionic neurons of the superior cervical ganglion. The superior cervical ganglion then sends the information through the carotid plexus and to the pineal gland [Retina -> Suprachiasmatic Nucleus -> Intermediolateral Column -> Superior Cervical Ganglion -> Carotid Plexus -> Pineal Gland].

Through these signals, the suprachiamastic nucleus is capable of setting the circadian clock. In the absence of these input, the internal circadian clock is actually 25,3 hrs long, instead of the usual 24 hrs that our day has. You might want to read the work by Turek et al. and Czeisler et al.

Regarding the basics of the suprachiasmatic nucleus, there are good reviews, I recommend an article by Moore RI (Organization and function of a central nervous system circadian oscillator: the suprachiasmatic hypothalamic nucleus) or even the book "Suprachiasmatic Nucleus: The Mind's Clock".

  • When you say you recommend these reads, from what perspective are you suggesting them? Medical student? College educated about biology? Layman? Oct 23 '15 at 22:19
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    I am a Medical Student with a great interest for neurosciences. The first two articles, by Turek et al. and Czeisler et al. are recommended in my neuroanatomy textbook and show how the light input from the retina is important in regulating the circadian cycle by the suprachiasmatic nucleus, and that in the absence of this input the circadian cycle progressively switches to 25.3 hours. The review by Moore is interesting and I have read it in the past. I did not read the book, but Moore is one of the authors, so I believe you'll find it useful as well.
    – Executer
    Oct 24 '15 at 9:13
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    Furthermore, perhaps you didn't think about it, but it's this mechanism that is also responsible for jet lag. The suprachiasmatic nucleus is usually synchronized with day and light. When you change timezones, because the clock is slow to reset (but it will, eventually), it gets desynchronized and that's why you can't sleep - it might be late, but your SC nucleus isn't synchronized with the new timezone yet. As time passes, the suprachiasmatic nucleus slowly adapts to the new timezone, thanks to the input from the retina. Article: Jet Lag (nejm.org/doi/full/10.1056/NEJMcp0909838)
    – Executer
    Oct 24 '15 at 9:21

From personal experience I maintained a sleep cycle of 6 hours a night for most of my 4 year college experience. I was tired initially when I switched to it, but felt normal after a week or 2. Some points to note about my experience:

  • The few times I did get a normal 8 hours of sleep I would feel better than normal for the next day or two.
  • I noticed decreased functionality in both my physical and mental state, which made tasks harder to do and/or take longer to complete.
  • I also learned that increasing water intake and decreasing energy drinks/coffee helped to maintain my energy throughout the day.

Most of the symptoms where manageable until I tried to drop down to 4 hours of sleep and started getting dizzy spells.

I don't recommend doing this as I've experience many of the symptoms of sleep deprivation listed here, most of which are not enjoyable.

  • Personal experiences aren't really applicable to my question, unless you also shortened your time awake correlating to light and dark of the day (ie, you were inside a building that was dark for 6 hours and light for, say...10 hours). I'm looking for proportionately short or longer days, which means both your awake and sleep time proportionately change, not that you got by on sleeping less and being awake more. Oct 23 '15 at 22:23
  • I didn't realize you were referring specifically to how the amount of light received during the day affects your sleep, which makes my answer mute. You might finds some answers looking into how the earth's tilt effects how long days and nights are for some locations near the poles.
    – Gibado
    Oct 24 '15 at 3:36

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