This came up in research on observing silence mentally and physically.
Niksen I think of as a kind of "mindless" rest while being awake.
I've noticed sleep doesn't accomplish the same thing, as I can carry tension with myself during sleep and wake up still agitated.
I've noticed light mental activity like reading, browsing social media, or watching a film, may not accomplish the same thing, as they can keep the mind in an active state which can also keep the body "on edge" (while the body may be resting in comparison with greater physical exertion like exercise).
Some people are promoting concentration and focus exercises from the eastern religions, like mindfulness and meditation for relaxation - I was trying to avoid that for religious reasons, but noticed niksen seems to be even different than some kind of concentration exercise as it is not really concentrating on anything.
When I was younger I think moments of niksen or "zoning out" were more common, but then at some point you start to want to make use of more moments when awake, and think that sleep can rest you - this is where things got off track for me personally, I used "rest time" as some kind of time that would keep my mind active, and thought sleep should be sufficient rest, so it would just carry over tension.
Even while taking a break in the sun or on a walk, I might think I should be mentally praying or thinking about something, which would agitate the restfulness of the activity, but I thought if I didn't think that I was then "wasting time" since I could rest during sleep. What I've found through "niksen" is a seeming need to not be thinking of anything, to not focus on anything, possibly to not be making much use of the body (or lightly exercising?), to get a kind of rest that's "niksen".
I've seen niksen described as: "mindless relaxation", staring off into space, "doing nothing with a purpose", not being lazy but resting and recharging, allowing the mind to wander but not engaging the thoughts, "putting your brain on low power mode", taking a time out, a kind of decompression, and an imitation of animals who often take a lot of time doing nothing.
Does anyone get a feel for this concept or have any thoughts on clarifying it or tips on observing it?