[Gravityloss]

You can look at the EEG spectrogram or calculate its spectral entropy. When you think hard, it resembles white noise and has high entropy. If you close your eyes and relax, it gets more regular and entropy goes down. I've done this myself. And we've known about alpha and delta waves etc for more than a century. When you fall asleep, the signal goes more towards a sine wave, ie less entropy.

Many of these complexity or information content metrics work in similar ways. You can even turn EEG recordings from medical operations with anesthesia to a sound and listen to them. The frequencies are of course very low so you must speed it up. You can clearly say when the person fell asleep. A waterfall turns into a hum.

It makes sense, if you're making measurements of a complex system, of course white noise like results are more probable if there's actually something significant going on, than a simple sine wave. Or picture the modem calling sound. The low data rates are simple waves but the high data rate resembles white noise.