Mental time travel to the future might be reduced in sleep

•Physiological evidence suggests that memory for the future decreases in sleep.•Blind, independent raters analysed mentation reports from different states of consciousness.•References to memories for the future were quantified in the mentation reports.•Mental spaces analysis served as the pre-validated linguistic tool.•Reports from REM and non-REM sleep held relatively few references to memory for the future.

We present a quantitative study of mental time travel to the future in sleep. Three independent, blind judges analysed a total of 563 physiology-monitored mentation reports from sleep onset, REM sleep, non-REM sleep, and waking. The linguistic tool for the mentation report analysis is based on established grammatical and cognitive-semantic theories and has been validated in previous studies. Our data indicate that REM and non-REM sleep must be characterized by a reduction in mental time travel to the future, which would support earlier physiological evidence at the level of brain function.