Why are you supposed to take out contacts every night? Are there detrimental health effects, either short-term or long-term, to leaving them in for days at a time, sleeping with them on, etc.?
I’ve gone camping with mine a few times and left them in the whole time for lack of a mirror and sink; squirted some solution onto them in the morning and everything seemed fine.
So why the suggestion/instructions to remove them and soak them every night?
What I’ve been told is that the contacts reduce the amount of oxygen getting to the cornea(?) of the eyes, and your eyes will grow tiny little veins into the area to try to supply oxygen, and these can eventually occlude your vision. There’s a name for this condition, but I forget.
When I first started wearing contacts, almost 40 years ago, the Doc said that I could wear them for 2 or 3 weeks between cleanings. And I did.
A few years later a different Doc told me that I had tiny little veins growing into areas that they shouldn’t, and I had to take my contacts out every night to stop that or my vision would be affected. In fact, he made me switch back to my glasses for a few weeks and rechecked my eyes before he would let wear my contacts again, with the aforementioned caveat.
I’m sorry if I’m not very clear on the details, this was several years ago. At any rate, I now take my contacts out every night.
Well, for me I just wake up with my eyes feeling like somebody just sandblasted them, so that’s been good enough to discourage me. The optometrist basically explained to me what Bumbazine said. Don’t they still sell “extended wear” lenses that don’t require to be changed every night?
Contacts have gotten a lot better in recent times. I hate taking my contacts out after anything less than two weeks. I’ve worn them that way for about 10 years now, including about a year where I wore month-long contacts. Although I do have noticeable veins in my eyes, they haven’t been a problem yet.
That said, it is absolutely healthy to let your eyes have some rest for the aforementioned reasons. I always try to give my eyes at least a day without contacts between pairs.
Of course, the caveat is that you want to pay attention to what the specific contacts are rated for. Wearing daily contacts for a week straight will get you in real trouble.
You only get one pair of eyes. If there’s any chance of infection, irritation, or conjunctivitis, I wouldn’t get lazy and push my luck. I wore some years ago HARD contact lenses that had to be disinfected and soaked every night. Once I was on a plane trip and couldn’t take them out. I got conjunctivitis and it was no picnic! 
I’ve only slept in my (soft, non-extended-wear) contacts once. I woke up with horribly uncomfortable eyes and some sort of film over my vision that lasted most of the day. Scared the crap out of me!
The main problem is indeed oxygenation. One short term problem with a lack of oxygenation is that it will favour the growth of anaerobic bacteria on the cornea. Hence a risk of infection problem.
‘Permanent’ contact lenses are permeable to oxygen and can be worn all night.
Many of the more recent disposables have been designed to allow much more oxygen through than earlier models did. I sleep with my contacts in for weeks on end; I usually just put some rewetting drops in in the morning.
The problem Bumbazine brought up is called corneal neovascularization. It is indeed caused by lack of oxygen to the corneas for an extended period of time. I don’t even sleep in mine, and never have, but apparently wearing my contacts for 16+ hours a day was enough for me to end up with this. Fortunately my eye doc caught this very early and I can still wear contacts. I just have to buy the ones that allow the most oxygen to pass through. I’m not saying this will happen to anyone who sleeps in their contacts, but, as salinqmind said, you only get one pair of eyes.
[d&r] S^G [/d&r]
IIRC, not only do you have the trouble with corneal neovascularization, but also an increased risk of corneal cancer associated with so-called “extended wear” lenses. As I recall, the increased risk was significant, being something like double the risk with day-only lenses, but small (being in the neighborhood of 1% - 2%).
I stopped wearing contacts when these numbers came out (in the 90’s sometime, that would be), since I hated putting them in each morning and taking them out each night, but refused to double my risk of cancer for being a lazy slob. 
I would agree with this. I used to ignore the instructions and wear the lenses for as long they felt ok. Ended up doing enough damage to my eyes that I pretty much had to stop wearing them at all except on “special occasions”. You mightn’t notice that they’re hurting your eyes but that doesn’t mean they aren’t.