You may be interested in György Buzsáki’s “Rhythms of the Brain”:

https://www.amazon.com/Rhythms-Brain-Gyorgy-Buzsaki/dp/01998...

“...self-emerged oscillatory timing is the brain's fundamental organizer of neuronal information. The small-world-like connectivity of the cerebral cortex allows for global computation on multiple spatial and temporal scales. The perpetual interactions among the multiple network oscillators keep cortical systems in a highly sensitive "metastable" state and provide energy-efficient synchronizing mechanisms via weak links.”

Buzsaki argues, in this book, that brain waves are the method that the brain uses to coordinate activity in different regions of the brain. (High-frequency waves coordinate the activity of small areas, low-frequency waves coordinate the activity of large areas.)

http://www.amazon.com/Rhythms-Brain-Gyorgy-Buzsaki/dp/019982...

If you're curious about brain waves, I highly recommend this book. It is fascinating.

The actual paper is here:

http://www.nature.com/nature/journal/vaop/ncurrent/full/natu...

I'm not convinced about the Reuters article's framing of things in terms of relaxedness - the paper doesn't seem to make any mention of emotional state.

We don't fully understand what theta oscillations do, or why there are oscillations throughout the brain. We know they're more present during activities like navigation and learning, but there are other, slower oscillations that are usually more associated with meditation and states of relaxation than theta.

This is an important finding though, because it throws more light on how oscillations relate to learning.

Buzsaki's Rhythms of the Brain is a great though dense resource for learning about oscillations in the brain:

http://www.amazon.com/Rhythms-Brain-Gyorgy-Buzsaki/dp/019530...

I worked at a neuroscience lab for a bit. I was focusing on EEG, the signals, oscillations, and emergent patterns in the brain. For this, they highly recommended I read Buzsaki's Rhythms of the Brain: http://www.amazon.com/Rhythms-Brain-Gyorgy-Buzsaki/dp/019530... .