After my hair transplants I slept slightly elevated, probably at 45 degree angle. I positioned the pillows to support my neck and head just below my ears. Some pillow will use the V shape pregnancy pillows or regular travel pillows.
After 4 days the grafts are secure, normal sleeping may resume. If you have strip, you may want to sleep at same angle for longer or just be careful when laying down as sometimes the pressure may give you some discomfort along the donor closure.