Humanity’s tormented 21st-century relationship with sleep raises a great many questions. Most can be dealt with as follows: no, not nearly enough; yes, but that would require a wholesale transformation of lifestyle and socio-economic context; OK, but that seems extortionate for a mattress. Yet the most basic question of the lot — how does tiredness work and what makes us sleepy? — remains tauntingly and magnificently unanswered. Cracking its secrets, says the neuroscientist who has arguably come closest to doing so, could transform the waking and sleeping world as we know it. Masashi Yanagisawa and his team have taken a bigger step towards solving the puzzle than anyone yet, but it remains (for now, at least) a flat-out mystery. Anyone who says otherwise is probably trying to sell you something.