From the natural "sleep aid", why do the sound of waves and bonfires help sleep?

The sound of waves and bonfires belongs to white noise and has a calming effect on human beings.

In the early days of human development, most people lived in caves. At night, in order to avoid the attack of wild animals (and to keep warm), they will light a bonfire where they sleep. When they fall asleep with the sound of the bonfire, they will feel safer, so they will sleep more soundly. Over time, this gene has been preserved, and hearing these sounds will help them fall asleep.

Another reason is that these white noises have low sound, low energy content, no irritation to ears and regular sound. These factors add up to help you sleep.