NREM sleep has four levels or stages. Stage 1 sleep, the lightest stage, is the transition from being awake to deeper sleep. Stage 2, intermediate sleep, accounts for 40% to 50% percent of your sleep time. Stages 3 and 4, called slow wave or delta sleep, are the deepest levels and occur mostly in the first third of the night. It is during delta sleep when your body heals itself. It is also difficult to awaken from delta sleep, as most of us feel dazed or groggy.