dd2 had bronchiolitis at 4 weeks, and I went home for 2 hours while she was apparently asleep and fine, to pack a case and because it was dd1's 3rd birthday: I wanted to see her if only for an hour to cut cake, open pressies etc.
When I got back, there was a crowd of panicking doctors around her cot...her heart rate had dropped very low, and she was put on C-PAP and moved to HDU. Its taken me two years to recover from the shock, and I've often harangued myself for doing the wrong thing...it was awful. BUT I had no experience, no way of knowing things were going to go badly, the nurses assured me she'd not even know I'd gone (she probably didn't, she was too sick, though I didn't realize it) and that anyway I needed an overnight case.
She was in for a week, and I stayed in the hospital except for a few hours per day when dh would take over, and I would rush home to see dd1 (it was Christmas). I also saw what Blu saw - parents of longterm sick babies taking much needed time away. The babies were fine with the nurses, and tbh - from what I observed of dd2, so was she. At 4 weeks and ill, I don't think she cared much who was stroking her cheek, and the nurses in HDU were mostly lovely, loving, and very attentive.
But I couldn't stay away all night, personally - I was too scared. I stayed in the Patient's Hospital (perk of HDU) and expressed breast milk every 3 hours, it made me feel better and hopefully helped her.
Once she was back on the ward, in a side room, there was NO way I could leave her - except for a moment when she was asleep (and on monitors). The ward is different from HDU, where you have 24 hour non-stop care and only 2 babies per nurse.