No, sideways is not a comparative form of an adjective.
positive
The degree of comparison for "sideways" is the positive degree, as it simply describes the manner in which some people sleep. It is not being compared to anything else in this sentence.
It does not go sideways!
can you walk sideways
He parked his car sideways in the crowded parking lot.
You can make the view sideways by putting the Game Camera sideways on something.
move the cyclic sideways
It is automatically played sideways...
The Sideways Door was created in 2006.
South of Sideways was created in 2003.
Sideways was released on 10/22/2004.
The Production Budget for Sideways was $17,000,000.