Much like stage 3, this stage is characterized by Delta wave brain activity. Over 50% of your brain waves are Delta waves, but occasional bursts of higher activity still occur. It is very difficult to rouse a person from stage 4 sleep. Interestingly, it is during stage 4 sleep that most cases of sleepwalking, night terrors, and even bedwetting occur. Stage 4 sleeps lasts the longest in the early part of the night, but gradually decreases in length as the night progresses, until it nearly disappears by early morning.