Why do humans (and other animals I believe) shut their eyes when they sleep?
Shut our eyes as opposed to keeping them open constantly, or blinking normally?
open continually. (BTW, best I can tell, the Master never answered this)
If our eyes were open continually, they would dessicate. Not good. I think some animals have a third, transparent eyelid that helps with the drying issue. Perhaps this eyelid is activated when they are asleep.
People do sometimes fall asleep with their eyes open. However, apart from the dessication issue mentioned by IceQube (which could be ameliorated by occasional blinking during sleep), I imagine that having them closed prevents bugs and dirt getting into them. (When you are awake, you can blink or turn away before the bug gets there.) Also, it is difficult to sleep when there is too much sensory stimulation, and closing your eyes helps cut down on that. It is that for that reason, I suppose, that falling asleep with one’s eyes open is rare, and not very long ago we had a thread asking how it is even possible.
REM - During sleep the eyes go through a stage of rapid movement. This is a critical stage of sleep. I don’t think it is fully known why this happens.
This is true, but why is it relevant? The eyes are rapidly on the move (in a very similar way) nearly all of the time when we are awake too.
For me, at least, closing my eyes shuts out a whole lot of visual stimuli, and makes it much easier to fall asleep.
Some prey animals will sleep with their eyes open. I’ve seen our chinchillas do this. I assume that even while asleep they blink often enough to keep the eyes from drying out, and wake up if anything gets close enough to their eyes to pose a danger.
REM is distinctly different from regular eye movement. Once again, the last I had read, was that it is very essential but the " why" is not understood.
It’s so birds and insects don’t eat our eyes while we’re asleep.
No it isn’t. Differences are minor to non-existent. See, e.g.:
Herman, J.H., Erman, M., Boys, R., Peiser, L.,Taylor, M.E., & Roffwarg, H.P. (1984). Evidence for a Directional Correspondence Between Eye Movements and Dream Imagery in REM Sleep. Sleep (7 #1) 52-63.
Leclair-Visonneau, L., Oudiette, D., Gaymard, B., Leu-Semenescu, S., & Arnulf, I. (2010). Do the Eyes Scan Dream Images During Rapid Eye Movement Sleep? Evidence from the Rapid Eye Movement Sleep Behaviour Disorder Model. Brain (133) 1737-1746.
Sprenger, A., Lappe-Osthege, M., Talamo, S., Gais, S., Kimmig, H., & Helmchen, C. (2010). Eye Movements During REM Sleep and Imagination of Visual Scenes. Neuroreport (21 #1) 45-49.
Hobson, J.A. (2009). REM Sleep and Dreaming: Towards a Theory of Protoconsciousness. Nature Reviews: Neuroscience (10) 803-813.
Doricchi, F., Iaria, G., Silvetti, M., Figliozzi, F., & Siegler, I. (2007). The “Ways” We Look at Dreams: Evidence from Unilateral Spatial Neglect (With an Evolutionary Account of Dream Bizarreness). Experimental Brain Research (178 #4) 450-461.
Hong, C.C.-H., Gillin, J.C., Dow, B.M., Wu, J., & Buchsbaum, M.S. (1995). Localized and Lateralized Cerebral Glucose Metabolism Associated with Eye Movements During REM Sleep and Wakefulness: A Positron Emission Tomography (PET) Study. Sleep (18) 570-580.
Hong, C.C.-H., Harris, J.C., Pearlson, G.D., Kim, J.-S., Calhoun, V.D., Fallon, J.H., Golay, X., Gillen, J.S., Simmonds, D.J., van Zijl, P.C.M., Zee, D.S., & Pekar, J.J. (2009). fMRI Evidence for Multisensory Recruitment Associated With Rapid Eye Movements During Sleep. Human Brain Mapping (30 #5) 1705-1722.
Hong, C.C.-H., Potkin, S.G., Antrobus, J.S., Dow, B.M., Callaghan, G.M., & Gillin, J.C. (1997). REM Sleep Eye Movement Counts Correlate with Visual Imagery in Dreaming: A Pilot Study. Psychophysiology (34) 377-381.
That may be so, but, once again, what is the relevance of this to the OP’s question about why we (usually) shut our eyes to sleep?
Fish sleep with their eyes open. Of course, they don’t have to worry about their eyes drying out.
As for eliminating sensory distractions, if you sleep in a darkened room, that shouldn’t matter.
But… the real reason to close your eyes while you sleep, is so the boogie man won’t get you.
Keeping your eyes open requires a certain musculature, and those muscles can rest with your eyes closed. Which is why, when you get sleepy, your eyelids “fall”—you stop using the voluntary muscles that keep them open.
It would be possible to sleep while keeping your eyes open, just as it would be possible to sleep holding an object in your prehensile hand, which also requires the use of certain muscles not at rest. But under ordinary circumstances, your body prefers to sleep with muscles unflexed.
Possibly also to help minimize dilation of the pupils while sleeping.
Interestingly if you hyper stimulate your senses, including your vision, you can fall asleep within minutes even with your eyes open.
I’ve spent ten minutes trying to find a cite for this online but can’t, so you’ll have to trust me. There was an experiment done where they subjected volunteers to rhytmically flashing lights, and rhythmic tones while forcing their eyelids to be open, they all entered REM rapidly.
- In the movie The Hollow Man, Kevin Bacon is made invisible - including his eyelids. He complains that he can’t sleep, because even when he closes his eyes, he can still see thru them. This would jibe with my experience of trying to snooze on the beach on a nice summer day - even with eyes shut, ambient light makes it much harder (tho not impossible - when exhausted, I’ve fallen asleep in an armored vehicle with lites on). I guess we can retrain ourselves, but it’s probably hardwired into our genes to block out light when trying to sleep.
- Under the influence of strong narcotix (like Dilaudid), there are memory gaps, times when I ‘know’ I didn’t sleep but time seemed to warp. The official party line is that the Narcos are only painkillers not knockouts, but they also ain’t supposed to be memory-killers either. Do some people go to sleep with eyes open under influence of strong narcotix?
Cats also sleep with their eyes open; according to their brainwaves, a cat lying in wait to pounce on something is often in fact asleep with their eyes open. Apparently they can basically just put their “pounce” reflexes on automatic and catch a nap while they wait for the mouse or whatever to show up.
Also, fish have no eyelids, so they don’t really have a choice.
I believe snakes also have no eyelids, though I don’t know how they stop their eyes from drying out.