One disorder that contributes to this list deserves special notice: obstructive sleep apnoea. This is commonest in middle-aged men who may be overweight but who, in particular, have large necks. In a serious form, it may affect up to 3% of men. It involves airway collapse on inspiration during sleep. This typically happens when sleeping at night lying on the back. Airway collapse stops any breathing until the respiratory drive becomes so intense that the airway is forced open – usually with a loud snort. The effort is so intense that the individual usually has their sleep disturbed, leading to poor-quality sleep and hence to tiredness the next day. The snort is so dramatic and loud that bed partners are often woken. The diagnosis is therefore often made by interviewing the sleeping partner, who complains about snoring. They will usually have noticed that their partner often appears to stop breathing for anything from 10 to 60seconds. The significance of this condition is that poor sleep and fatigue the next day may lead to requests for something to improve sleep – but treatment with hypnotics may be fatal. The condition can be treated successfully with devices delivering continuous positive airway pressure (CPAP).

There are two other notable but relatively rare conditions that are partly physical and partly social: advanced sleep-phase insomnia and delayed sleep-phase insomnia. In advanced sleep-phase insomnia individuals fall asleep too early in the evening and wake too early, while in delayed sleep-phase insomnia, they fall asleep too late and are then unable to get up the next day. These disorders stem from the functioning of the circadian clock. Essentially we all tend constitutionally to be either ‘larks’ (waking early and at our best early in the day) or ‘owls’ (at our best later in the day or in the evening). Advanced and delayed sleep-phase disorders are exaggerations of these tendencies that may require specialist help to correct.