Half a tennis ball over each eye

The brain constantly hunts for differences in the incoming signal. Remove all the differences — flat light, flat sound — and after a while it starts filling the picture in by itself. The effect was described as the Ganzfeld back in the 1930s and has been reproduced in labs ever since: it isn't mysticism, it's how the visual system behaves with no input to work on.

⚠️ This quest is not for people with a history of anxiety, panic attacks, dissociative states or psychotic episodes. If in doubt, skip it. Stop the moment it feels unpleasant — this is not an endurance test. Cut a tennis ball in half and place one half over each eye — the rim should rest on your cheekbone and brow without pressing. Lie or sit so your face is under even, diffuse light: a desk lamp through white fabric, or a window on an overcast day. There should be no sharp shadows. Put on quiet white noise or rain sound through headphones. Then simply lie there with your eyes open for half an hour and expect nothing. Set an alarm so you\'re not checking the time. The first ten minutes are usually boring — that\'s normal. Come out slowly: take the halves off and sit for a minute in ordinary light before standing up.

At what point did you stop being able to tell whether your eyes were open or closed?