Q. After performing the salatul sunnat, if we were to sleep on our sides, do we have to renew our wudhu' (ablution)?

A. Yes, sleeping on one's side cancels wudu'. If someone remained sitting and slept, he keeps his wudu'. One of the special characteristics of the Prophet, blessings and Peace upon him is that even his deep sleep while lying on his side did not cancel his wudu', but this is a special attribute for him, and Allah knows best.