Severe apnea. I have been using a CPAP since 2007. About 3 years ago it broke and I had to go without for about 3 weeks. I was miserable! (So miserable I bought a spare.)
It's a nuissance - hoses, masks, cleaning, distilled water, etc. - but a small price to pay for the glorious sleep I get.
I remember waking up in the sleep clinic thinking, I slept better than I thought I would only to have the technician walk in and say Dude, you've got sleep apnea! You didn't sleep at all! (He really said Dude) I had no idea how poorly was sleeping until I was sleeping better.