Most of the ICU areas I've seen have very strict visiting hours. They will usually allow one or two people back at a time for a certain length of time, and then the unit is closed for, say two hours, and there are no after hours visits unless the patient turns critical.
There is nothing more awful than getting an early morning phone call when you have a loved one in ICU. It's almost always very bad news.
I don't know how ICU nurses do it, since they have to be on the unit all the time, but I certainly take my hat off to them. They have an incredibly difficult job, but most of them are fantastic. I salute them.