Self-generated optic flow activities like walking, running, or cycling can serve as a form of non-sleep deep rest by shutting down brain areas involved in alertness.

“When we are in a mode of self-generated optic flow like walking or running or cycling and things are just floating past us on our retina, we're not really looking anywhere in particular, so this is the opposite of a tight window of focus. When we do that, there are areas of the brain like the amygdala which are involved in releasing epinephrin and create alertness. At the extremes, it creates fear but certainly alertness, those are all shut down. So it's its own form of non sleep deep rest.”