Personally, I don't believe that I could cuddle with a robot - but I'm probably not the target audience for Somnox the sleep robot.
(Somnox sleep robot video)
Somnox is the first robot you might actually want to sleep with. To be fair, it looks more like a pillow than a robot, but it's equipped with an accelerometer, an audio sensor and a carbon dioxide sensor. Hold it to your chest when you're trying to fall asleep, and you'll actually feel Somnox expand in and out as though it's breathing. It's meant to help you relax, regulate your breathing with its own and fall asleep.
Due out this fall for $550 (roughly £400 or AU$700), the Somnox can use its carbon dioxide sensor to tell when you fall asleep, and it'll shut itself off. I'm skeptical that feeling something else breathing would help me fall asleep, but a colleague of mine tried it out for a night and got a great night's sleep.
I'm pretty sure that in Millennium, John Varley's sex robot Sherman would also monitor sleep, but I can't find my copy.
If you're interested in science fictional sleep aids, check out the sleep set from Ringworld (1970) by Larry Niven, the hypnoteleset from The Space Merchants (1952) by Pohl and Kornbluth and sleep surrogate from Methuselah's Children (1941) by Robert Heinlein.