For people with non-24, bedtime creeps later every single day
Most body clocks run slightly longer than 24 hours, and morning light nudges them back into line each day. Without that reset, sleep drifts steadily around the clock. That is life with non-24-hour sleep-wake disorder, which affects up to 70 per cent of totally blind people and, more rarely, some sighted ones.
The internal clock sits in the suprachiasmatic nucleus of the hypothalamus and needs light arriving through the retina to stay synchronised. Remove that signal, as happens in total blindness when light-sensitive retinal cells are lost, and the clock free-runs at its own pace. Peak alertness, the daily low in body temperature, metabolism and hormone release all slide a little further each day. People who keep even a trace of light perception rarely develop the condition, and a few patients who are subjectively blind still entrain normally because the pathway from retina to hypothalamus survives.
In sighted people the causes are murkier and appear neurological. One case followed a head injury, another involved a large pituitary tumour pressing on the optic chiasm, and several followed chronotherapy, a treatment for delayed sleep phase disorder that deliberately lengthens the sleep-wake cycle. Researchers also describe patients who seem unusually unresponsive to bright light, and whose temperature low comes so early in the night that morning light has little effect.
Numbers show how far off these clocks run. In a 2002 study, five sighted patients averaged a cycle of 25.12 hours, against 24.02 hours for healthy controls. A 2007 review put the typical circadian period of people with the disorder at 24.8 hours and their usual sleep at nine to ten hours. The condition often cycles, with weeks or months of daytime sleeping alternating with spells when sleep lines up with the night.
Living against the drift is exhausting. Forcing a standard schedule tends to produce insomnia and crushing sleepiness, even microsleeps, along with headaches, low appetite and low mood. Many cannot hold conventional jobs or keep up at school, and the disorder is described as an invisible disability. Some choose to follow their shifting clock instead, which improves sleep at a heavy social and professional cost.
Source: Non-24-hour sleep–wake disorder
