what stage of sleep is considered deep sleep

Deep sleep is also known as stage 3 or N3 sleep. During deep sleep, electrical activity in the brain appears in long, slow waves called delta waves. It is the deepest stage of non-rapid eye movement (NREM) sleep. In this stage, it is harder to wake someone up, and if someone woke you up, you would feel disoriented for a few minutes. During the early sleep cycles, N3 stages commonly last for 20 to 40 minutes, and you spend the most time in deep sleep during the first half of the night. As you continue sleeping, these stages get shorter, and more time gets spent in REM sleep instead.