…but we don’t have any bodily way to “turn off” (for want of a better phrase) our other senses? I mean, why don’t we have flaps of skin to cover our ears and block out hearing? Or something that slips over our tongues to eliminate taste? Or smell? Or touch?
And for that matter (and this is really the thought that led to the above question), why is it necessary to close our eyes in order to sleep?
Thanks…
<wag>I think the ability to close our eyes is intended to protect them, not motivated by some need to turn off our ability to see.</wag>
It’s not necessary to close our eyes in order to sleep, it just makes it easier because it removes a great source of distraction. I’ve fallen asleep with my eyes open several times. Snakes, I think, have no eyelids, and they (apparently) manage to sleep just fine.
I apologize in advance for the hikack, but just can’t help myself:
Why does “how come” mean “why”? Anyone know the history of that idiomatic expression? I’ve always wondered…
The Columbia Guide to Standard American English has this to say:
Back to the subject at hand, though. You can “turn off” your sense of smell by pinching your nose shut. It’s effectively turning off your vision in the same manner. Your retinas are still doing their job at the back of your eye whether you close your eyes or not, much like your nasal receptors are still eagerly awaiting a sniff whether your nose is blocked or not.
I know my son can turn off the ability to hear me…
I wish men had the natural ability to voluntarily control the release of sperm during orgasm.
You mean you don’t? 
Many semi-aquatic animals can shut their nostrils and ears at will.
I fall asleep with my eyes open all the time. Babysitters used to think that I was dead when I slept because my eyes are wide open and roll back. :eek:
People have even had conversations with me, since I sometimes talk in my sleep as well. When I sleep, I appear somewhat coherent but I don’t remember anything later. Interestingly, the same sort of thing happens when I play video games. 
Sight: You close your eyes. You still “SEE” (try it sometime) but most external influence is blocked by a thin layer of skin, pulled in place by the muscles of your body, at your own conscious will.
Smell: You pinch your nostrils shut. Your smell receptors are still working, but no new smells can enter. You do this with the muscles of your body (fingers) at your own conscious will.
Hearing: You put your fingers in your ears. This blocks most external noise, but your ‘ears’ are still working.
Taste: You close your mouth tightly, so nothing can get inside. The tongue is still there, but no external stimulii enters.
Touch: Ok. Beats me. This is a tough one. Can you count ‘just standing in once place without touching anything’ as ‘turning it off’? I dunno. I do know that I and a few other people I’ve met have some conscious control over it, in that we can ‘will off’ our tickle response. If I don’t want to feel someone tickling me, i just decide it doesn’t tickle. I still feel the touch, however, but it doesn’t make me laugh.
Ejaculation: Well… TMI maybe, but there’s something called ‘tantric sex’. I don’t claim to be an expert in it by any means, but there is a way to use the fingers to block the Vas Deferens (Sp?) during orgasm, which diverts semen into the bladder. It also feels REALLY good. q;}
I agree that the ability to close your eyes has nothing to do with turning the “sense” of sight off. We use our eyelids to clean the surface of our eyes every time we blink. That’s their primary function. Don’t reptiles have special eyelids to protect their eyes as well? I seem to remeber some snakes having an opaque lid that could cover their eyes or wipe them off, I’m not sure.
<WAG>
Its a self defence thing - we can afford to lose one sense whilst sleeping, but losing all of them would leave us incredibly open to danger.
</WAG>
I would say that, from a defense standpoint, it’s bad enough losing just one sense witjout losing all of them. Al though I do note that I don’t hear some things when asleep that I would hear if I were awake.
I can still see a bit with my eyelids closed - if there is a bright light (like the sun) shining on my eyelids I see a reddish glow. So our eyes aren’t completely turned off.
Our eyes need to blink every now and then to stop our eyes from drying out, etc, but when we’re sleeping there is no need for them to blink - so we just keep them closed. That also would conserve some energy and allow our eyelid muscles to relax.
Most reptiles have normal, fleshy eyelids, the same as we do, and can close their eyes whenever they want to. Some also have a secondary “eyelid”, called a nictating membrane, which is clear (esentially) and acts as a sort of windshield wiper as well as see-through protection. Snakes don’t have eyelids of either type. Snakes have clear, not opaque, coverings for the eyes that are fixed in place, called spectacles, that are really just specialized scales and are shed with the rest of the skin (ideally). The spectacles appear to be opaque just prior to shedding, due to the secretion of fluids under the outermost layer which softens and separates the outermost layer from the underlying layer to make it easier to shed. This does have a negative effect on the snake’s vision, which tends to make the snake cranky, but doesn’t completely blind the snake as an opaque covering would. I imagine it’s similar to us snorkeling in murky water; there’s some visibility, but details are lost. This phase typically only lasts about three days, though it varies. And, yes, this means snakes always have to sleep with their eyes open. It also makes it hard to tell whether a snake is asleep, zoning out, or staring at you.
I don’t know about anyone else, but when I’m asleep I don’t necessarily wake up because of sound, but because of changes in it. <— made no sense, here’s an example. I’ve often fallen asleep on the couch with some music (rock, so it’s not Classical soothing me to sleep or anything) playing at a pretty decent volume, slept fine, and then as soon as the CD ends, wake up. Getting back to the self-defense mechanism, I’d assume that if any sense detected a change, you’d wake up again. The appearance or disappearance of a foul stench, bright light, noise, touch or taste…don’t want to think about where that could come from.