Abstract
Sleep improves learning and consolidates memory. While this view is widely accepted, the notion that sleep can affect our perceptions, the way we view the world around us, has not yet been investigated. Here, we examine if sleep has an effect on visual perception, specifically on classification of stimulus color. On a given trial, a full-field homogeneous stimulus of either slightly reddish or greenish hue was displayed. The observer had to judge if the stimulus was greener or redder than their internal percept of neutral gray. Across trials, the hue was varied using the method of constant stimuli. One pair of monocular tests was run just before the observer went to sleep overnight and the second pair immediately after the person woke up. Sleep duration was monitored with sleep diaries and actigraphy (7.7 hours on average). A comparison of pre- and post-sleep data (n=5 observers) yielded a small but significant change: After sleep as compared to before, the stimulus was significantly less likely to perceptually take on a greenish tint (p<0.01, bootstrapping statistics). A closer look at the results reveals that it is not sleep that causes gray to be classified as reddish, but prior wakefulness that causes gray to be classified as greenish and sleep restores perception to achromatic “equilibrium”, i.e. following overnight sleep, physical gray is perceived as gray. Overnight full-field monocular stimulation of a flickering red ganzfeld failed to nullify the recalibrating sleep-induced effect: An achromatic stimulus was still less likely to be classified as greenish following sleep (n=9), with no statistical difference in the magnitude of the recalibration in each eye. This suggests that recalibration is an obligatory, internal, sleep-dependent process that external stimulation cannot modulate. Our tentative conclusion is that wakefulness causes color classification to drift away from neutrality and sleep restores it back.