The Biological Basis of Chronotype
Chronotype is the tendency in a person's patterns of activity and alertness that follows from their circadian rhythm, and it is broadly divided into morning types (morningness, the lark), evening types (eveningness, the owl), and intermediate types. Chronotype reflects individual variation in the body clock centred on the suprachiasmatic nucleus (SCN), and genetic factors account for roughly 50 per cent of that variation. Polymorphisms in clock genes such as CLOCK, PER, and CRY all contribute to individual differences in chronotype. Morning types are highly alert in the early hours, reach their performance peak before noon, and grow sleepy early in the evening. Evening types become more alert from the afternoon into the night, stay active until late, and find waking in the morning difficult. Chronotype also shifts with age: it moves toward eveningness during adolescence and tends to revert toward morningness as a person grows older. Measurement is usually carried out with the Horne-Östberg Morningness-Eveningness Questionnaire (MEQ) or the Munich Chronotype Questionnaire (MCTQ).
Chronotype Mismatch and Its Effects on a Relationship
A mismatch of chronotypes within a couple is treated as a factor that can affect the quality of the relationship. Couples whose chronotypes align are said to tend toward higher relationship satisfaction, and higher sexual satisfaction as well, than couples whose chronotypes diverge, although conditions on the practical side of life, such as working hours and the division of childcare and housework, are bound up in this as well, so satisfaction is not settled by the alignment of rhythms alone. Several mechanisms carry that effect. (1) Reduced shared time: when one partner is at their most active while the other is sleepy, high-quality time together is hard to secure. (2) Mismatched bedtimes: the habit of going to bed together contributes to the maintenance of intimacy, and differing chronotypes make it difficult. (3) Mismatched sexual timing: the hours at which sexual desire peaks are not the same for both partners. (4) Friction in daily rhythms: a morning type wakes an evening type with the noise of getting up early, an evening type's late hours disturb a morning type's sleep, and similar everyday abrasions accumulate. A chronotype mismatch is not, however, an absolute barrier; it can be managed through deliberate adjustment, such as protecting shared time and respecting each other's rhythm.
Chronotype in Compatibility Assessment
Chronotype is a usefully clear-cut thing to work with, precisely because it can be kept separate from questions of personality. As for its relation to the Big Five, evening types have been reported to be somewhat higher in openness and extraversion and somewhat lower in conscientiousness, though the effect sizes are small, so it is a stretch to try to read someone's character off the hour at which they get up. What genuinely helps is something else: the way chronotype lets you relocate the cause of a mismatch from a person's character to a time of day. Once you understand that the one who will not get up in the morning is not being lazy but simply has a body clock that has not yet entered its waking phase, you have one fewer thing to hold against them. From there, the arrangements worth settling in advance are concrete ones: which part of the day the two of you will spend together, such as keeping weekend mornings separate and doing things together from the afternoon; how you will say goodnight on the days you go to bed at different hours; and how much noise from an early riser, or from someone still up late, each of you will put up with. A difference in rhythms is better handled as an area that calls for arrangements of that kind than as grounds for concluding that you are poorly matched, because arrangements are something you can actually act on.