The worst part for me was the difficulty sleeping during the distraction phase, though the "worst part" varies with patients. When a patient is lengthening, your body needs a lot of rest and sleep to heal (i.e. build up callus and then bone formation, as well as 3 weeks from surgery). However, you are stuck sleeping on your back, because it's very difficult to shift your body while sleeping. But your body naturally shifts around while sleeping to get comfortable. So about every 2 hours, I would wake up with muscle stiffness and poor circulation, then stretch to relieve the tightness and get better circulation for several minutes, then go back to sleep.
At least 2 hours of sleep was enough to get 1 REM cycle of sleep. Around 7 to 8 weeks after surgery, I managed to sleep on my side for some variety. Then when I finished lengthening, I noticed my required sleep started to slowly and steadily decline a few weeks after to more normal levels, I was sleeping longer (like 4 hours or more) before waking up, and my sleep quality improved.