Can humans acclimate to longer or shorter days?
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.
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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".
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.
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