Тест на психическую и физическую усталость.

Иллюзия японского психиатра «Вращающийся змей»
http://fit4brain.com/7096/

Говорит спать пора.

Don’t get alarmed. For one, the effect depends on eye movements, and these are known to differ markedly between subjects without relating clearly to psychological traits. Further, a few people do not see it at all (could be around 5%, among them a very renowned vision scientist), in spite of appropriate eye movements. There are no actual data showing relations to stress (or age), so don't distress yourself when you see it rotating strongly or not at all.

http://www.michaelbach.de/ot/mot-snakes/index.html