The Clocks We Are Born With

AAsk people whether they are a morning or an evening person and almost everyone has an answer ready. Chronobiologists call the trait chronotype, and they measure it less by opinion than by clock time: the midpoint of sleep on days when nothing obliges a person to get up. The questionnaire that fixed the idea in the field was published in 1976 and is still administered, though most large studies now prefer the sleep-midpoint measure, which survives translation into other languages more gracefully. When Marta Ihlen's group applied that measure to 61,000 adults in 2019, the result was not the two tribes of popular writing. Fourteen per cent qualified as decidedly early and twelve per cent as decidedly late; the remainder were distributed between them, forming a curve with a long tail rather than a pair of camps. Larks and owls, Ihlen likes to say, are the ends of a rope that most people are standing somewhere along.

BThe trait has a physical seat. A cluster of some twenty thousand cells above the crossing of the optic nerves keeps a rhythm of roughly a day, which specialised receptors in the eye correct each morning by reference to light. Left in constant dim conditions, the average human cycle runs to about twenty-four hours and eleven minutes, and small differences in that period, compounded night after night, separate the person who wakes easily at six from the one who cannot. Ihlen's own summary of the twin literature puts the heritability of chronotype at 40 to 50 per cent, and several common variants, among them one in the gene PER3, have been shown to move sleep timing by twenty to forty minutes. Inheritance, though, does not act alone. In communities Ihlen surveyed in the Andes and in northern Ghana, where household electric light is weak or absent and the strongest timing cue available is daylight, the spread of chronotypes narrows to roughly half of that recorded in cities.

CWhat modern lighting is free to exaggerate is the subject of Ihlen's larger project. Her cohort of 27,000 adults, followed since 2014, records the gap between a person's sleep timing on working days and on free days, which she calls social jetlag. About a third of the sample carried a gap of more than two hours, and that group showed higher rates of metabolic disorder, depression and smoking than colleagues of the same age doing the same jobs. Her insistence is that the damage lies in the mismatch and not in the shortage: participants who slept few hours but at a consistent time fared better than those who slept longer at hours their employers had chosen for them. She is candid about the study's weaknesses. Sleep timing was reported by the participants themselves, with wrist recorders worn by a subsample of 1,100; and shift work, which she could not exclude from the cohort, may account for part of what she sees.

DDaniel Kwarteng, an epidemiologist at the Bergholt School of Public Health, has become the most quoted of the sceptics, though the position he holds is narrower than the headlines suggest. He does not dispute that chronotype is substantially inherited, nor that misalignment is unpleasant to live with. His objection is to the size of the effects and to what is built upon them. Reanalysing nine cohorts, Ihlen's among them, with adjustment for household income, shift work, smoking and years of education, he found the association between social jetlag and metabolic disorder reduced by about sixty per cent: smaller, but still present. A residue of that size, he argues, will not carry the weight of a national policy. The disagreement, he adds, is persistently misreported as a quarrel over whether body clocks exist at all.

ESchools are where the argument has been tested in public. In 2021 the Aldergrove district moved the start of its secondary day from 08:10 to 09:05 and agreed to be measured. Eight weeks in, pupils were sleeping forty-three minutes longer on school nights; by the ninth month, the last of the school year, the gain had fallen to twenty-two. Lateness dropped by a third and stayed down. Attainment rose by 0.06 of a standard deviation, a figure the evaluators declined to distinguish from zero. Both researchers claim the result. Kwarteng points to the attainment figure and to an expense the arithmetic usually omits: a district that moves its start must move its transport as well, and that bill is rarely counted. Ihlen reads the fading differently. The extra hour, she argues, was partly spent rather than banked, because pupils released from an early alarm drifted their bedtimes later still, under evening light that no timetable governs.

FSome things both accept. Chronotype is not fixed for life: evening preference strengthens throughout adolescence, peaks at about nineteen, and then retreats year by year until the average sixty-year-old rises earlier than the average twelve-year-old. Neither has patience for the older habit of reading a late clock as a sign of idleness. Ihlen is dismissive of the popular account that blames screens, which she treats as one modest source of evening light among many, and holds that the workable remedy for most occupations is flexibility over starting times rather than exhortation. Kwarteng would go no further than permitting such flexibility where it costs an employer nothing. What now separates the two is not the measurements, which they largely share, but a question no cohort study will settle: what a society owes the people whose clocks it was not built around.