Sleep seems like something you can do from a neurophys fellowship if you've comfortable with EEGs. Many places offer 24 hour EEGs which obviously include sleep.
But sleep is its own fellowship as well and I know we've sent a resident into such a fellowship 2 years ago.
As you probably know, there's a lot of demand and money in sleep and you'd compete with pulmonary.
Fellowships run for 1-2 years depending what you want to get out of them.