Cohabitation between a night owl and a lark is a tricky business. No, we are not talking about zoo animals, but two people with two different lifestyles. A lark (or a morning person) wakes up at the first rays of dawn. Meanwhile, an owl (or a night person) often feels their best later in the day. When these two live together, the lark may start their morning in the dark, careful not to awaken the owl who, on the other hand, has their evening burst of energy met with the lark’s weariness.

These different patterns can lead to arguments, so one may ask: Can’t one person concede [1] and adjust their sleep schedule? In fact, being a night owl was once considered simple laziness. However, adapting one’s sleep pattern takes more than strong will, and linking a delayed sleep pattern to a character flaw is a huge misconception according to chronobiological studies. Yes, there is science—biology—behind this night- and morning-person distinction.

People have different “chronotypes,” which are natural tendencies regarding when they prefer to sleep. Larks, also known as “morning people,” have earlier chronotypes [2] than owls. This chronotype affects their physiological responses throughout the 24-hour cycle, giving rise to a particular circadian rhythm.

Studies have shown that chronotype is a heritable trait, pointing to a genetic basis. While circadian rhythms can be retrained, chronotypes are more hardwired [3] into our system. A 2017 paper published in PLOS One, titled “Chronotypes in the US,” looked at the distribution of chronotypes in the US population. Surprisingly, it revealed near-normal distributions: “Twelve years of pooled diary data were used from the American Time Use Survey. We observed a near-normal distribution overall and within each age group.” This can mean the lark-owl distinction is a natural phenomenon.

So, when you’re living with someone with a different chronotype, rather than forcing changes in either one of your sleep schedules, accepting the difference and negotiating around it might be the best bet for harmony. Mom would tell me to date a man who has a similar lifestyle—but if life happens so that you fall in love with a person who has the opposite chronotype, brace yourself for [4] future negotiations.