Although a universal necessity, sleep, as the past powerfully indicates, is not a biological constant. Before the Industrial Revolution, sleep in western households differed in a variety of respects from that of today. Arising chiefly from a dearth of artificial illumination, the predominant form of sleep was segmented, consisting of two intervals of roughly 3 hours apiece bridged by up to an hour or so of wakefulness. Notwithstanding steps taken by families to preserve the tranquillity of their slumber, the quality of pre-industrial sleep was poor, owing to illness, anxiety, and environmental vexations. Large portions of the labouring population almost certainly suffered from sleep deprivation. Despite the prevalence of sleep-onset insomnia, awakening in the middle of the night was thought normal. Not until the turn of the nineteenth century and sleep’s consolidation did physicians view segmented sleep as a disorder requiring medication.