The only time I have ever done this is when I was on a plane for 16 hours. My eyesight without my contacts isn't that good so on a plane when I might need to do something at random periods of time eg. Use bathroom, eat, etc I would have trouble without contacts. However, I took them off when I got to my destination and I had no problems whatsoever.
Last night I went to a friend's house for a sleepover, and I didn't take off my contacts. I figured it would be fine, and the box of my contact lenses said I could leave them in overnight for 6 days straight. (I use Accuve Oasis). I did not leave them in overnight prior to going to my friend's house.
This morning when I got back home my eyes felt okay but a little sore, so I took off my contacts to be safe. (I have extra glasses). I can see a little redness around the pupil of my eye. Is it something I need to be worried about? I'm not in extreme pain, and I can see fine.
Added (1). I only slept for about four hours, not sure if that makes a difference or not.
Read more: Left my contacts in overnight?