hi christiana, all 3 of mine have slept on their sides.
ds1 had breathing problems and all the midwives in the hospital said it was ok to put him on the side if it helped him breathe a bit better.
it's safer than on their tummy, though not as safe as on the back.
as long as you wedge them in so they can't roll.
ds2 was a nightmare baby and just slept better on his side. quite frankly if he had wanted to sleep hanging by one leg from the ceiling i'd have let him lol
and ds3 seems to prefer it too.
a good thing about it is that they tend not to get a flat spot on the back of the head, or a bald patch. you need to alternate sides though