> Our ancestores have been sleeping perfectly fine on hay and grass

Our ancestors also had very different environmental influences. Artificial light distorts our circadian rhythm and hormones. Stimulants such as caffeine also influence our sleep patterns.

Bi-phasic sleep was once very common but today's "normal" is thought to be a single uninterrupted block of sleep.

It's not to say that either (or indeed any other) approach is "normal" or "better" – just a recognition that human sleep patterns have a very diverse history.