Like most of our ape ancestors, we have really had only one response to the fall of night. We have stretched and yawned, we have climbed upwards, we’ve lain down somewhere soft, closed our eyes and shut the whole thing out until morning. Humans may have exchanged tree trunks for a set of stairs, and bunches of green leaves for sprung mattresses, but the same basic reflex has been ongoing among large primates for four million years.