There is a forgotten sunnah we should try to revive before we sleep. Prophet Muhammad (PBUH) said “when you go to bed, make wudu just like you make wudu for prayer.” 

Some scholars say that this is even the case if you already have wudu. Others say that the purpose is to sleep in a state of purity so if you are already in wudu it is OK, no need to make it again. 

Allah takes the souls at the time of their death and those that don’t die when they are asleep. Sleep is considered a state of uncertainty so it is important to sleep in a state of purity.