Submitted by SpoonKitty-chan t3_10j2xk6 in explainlikeimfive
By ancestors I mean like "cavemen" in a sense. If they had to sleep in caves surrounded by a million different noises, predators, insects, and sleeping on primitive beds - I just cant see them getting any good night rests.
In today's world sometimes we cant sleep even with great pillows, mattresses, soothing sounds, sleep aid medicine, etc. For some even the tiniest noise or light wakes them up. Thanks for ELI5 and whatever the reason early hominids must have been pretty hardcore.