For sleep is rather extraordinary. If I told you that I had a neurological disease which meant that for eight or more hours a day I lost control of my faculties, bade farewell to the outside world, and was subject to complex hallucinations and delusions – such as being chased by a grizzly bear at Stockport Railway Station – you would think I was in a pretty bad way. If I also claimed that the condition was infectious, you would wish me luck in coping with such a terrible disease, and bid me a hasty farewell.

Of course, sleep is not a disease at all, but the condition of daily (nightly) life for the vast majority of us. The fact that we accept without surprise the need for a prolonged black-out as part of our daily life highlights our tendency to take for granted anything about our condition that is universal. We don’t see how strange sleep is because (nearly) everyone sleeps. Indeed, the situation of those who do not suffer from Tallis’s Daily Hallucinating Delusional Syndrome is awful. They have something that truly deserves our sympathy: chronic insomnia.

Since all animals sleep, we assume it has a biological purpose. The trouble is, we don’t know what that purpose is. There are many theories – energy conservation, growth promotion, immobilisation during hours of darkness when it might be dangerous to be out and about, consolidation of memories – but they are all open to serious objections. William Dement, one of the leading researchers of the last century and co-discoverer of Rapid Eye Movement sleep, concluded from his fifty years in the forefront of the field that “the only reason we need to sleep that is really, really solid, is that we get sleepy.”

Raymond Tallis tackles an exhortation by Christopher Hamilton ​for philosophers to spend more time exploring the phenomenon of sleep.