As we go through the initial stages of NREM sleep, we become gradually less aware of our environment. This progressive tuning out may be adaptive because our brain is monitoring the world around us as we fall asleep, possibly to assess whether it is dangerous to sleep. Slowly receding perceptions of nearby friends and family talking, a crackling fire, infants crying, and the fact that those hyenas are far away signal to the brain that it is safe to enter a deeper, unconscious stage of sleep.
