No, not the late-lamented band, whom we praised a few weeks ago. We refer to the state in which the twitch of the closed eye betrays the unshackling of the imagination. REM is something shared with many animals. Humans can't even claim to do more than the rest – a glance at the REM league table suggests armadillos dream far bigger dreams. The unlikely connection between the flittering iris and the unconscious mind's eye was first discerned in the 1950s, and was established fact before long. Ever since, we have known we owe our nightly flights of fancy to this distinctive sleep phase which features a complex chemistry and irregular breathing as well as the rapid eye movement itself. We owe to it, too, the whole cultural story of dreaming which stretches from Sumerian myths to Freudian speculation by way of the Bible itself. Throughout, there's been speculation as to why we dream in the first place, and yet most of the myriad "theories" advanced remain just that. Now a paper in Current Biology sheds a little light on what happens in the dark hours. The researchers showed subjects images that pulled on the heartstrings before allowing half – and depriving the rest – of a proper sleep. The next day they saw the images again, and scans revealed that while the raw emotional centres non-sleepers brains still buzzed in response, the sleepers' reasoning apparatus kicked in. Sleep seems to lay demons to rest, or at least allow them to be approached in a dispassionate spirit. Sweet dreams indeed.