I learned about my friends' sleep problems by accident. We were having a cookout with three families not long ago, and the children were off playing by themselves. The couples sat down for an adult conversation that might otherwise have turned to Hollywood, parenting or Donald Trump, when suddenly one of the women announced she had a confession: She never got to see her husband.
She said she collapsed into bed soon after the children went to sleep, then woke up wired at 4:30am, anxious about work deadlines. He came home late from his job, played with the children for a time, then went to bed after 11pm.
Instead of finding this situation unusual, every other person at the table had a similar story. One spouse liked to meditate in the morning, another liked to binge-watch television at night; one liked reading when the house quieted down after midnight, another liked making coffee before the house got chaotic at dawn.
One thing they all had in common is that they had radically incompatible sleep schedules with their spouses. Another is that they weren't sure whether this was good or bad for their relationship.
In recent years, a consensus has emerged that sleep is a critical health issue, but researchers have largely focused on individual behaviour.
One area that has lagged behind is what researchers calls dyadic sleep, or sleep concordance. Sixty per cent of people sleep with another person. When one person has sleep issues, both can suffer.
Certain sleep disorders, like snoring, have been shown to reduce the quality of relationships, largely because the person hearing the snoring experiences disrupted sleep. Women living with snorers, for instance, are three times as likely to report sleep problems themselves. Insomnia has also been linked to lower relationship satisfaction.
Research into couples' sleeping patterns reveals a curious dynamic. When objective measures like brain waves or eye movements are examined, people are found to generally sleep better when they sleep by themselves than when they sleep with a bed partner.
Yet when they're asked about sleeping alone, people say they are less satisfied.
A chief impediment to sleeping together is different preferences for what time to go to bed. As early as the 1970s, researchers began looking at the distinction between morning people and night people, often referred to as "larks" or "owls".
Invented in 1976, the Morningness-Eveningness Questionnaire became a popular self-assessment that uses 19 questions to help determine what time of day a person's alertness peaks.
More recent research has shown the variance is largely determined by genetics, with some input from age and gender.
Till Roenneberg, a professor of chronobiology at Ludwig-Maximilian University in Munich, studies the biological roots of sleep. He told me that each person has a sleep chronotype, an internal timing profile that is specific to that individual and can vary up to 12 hours with others.
When I asked how many different chronotypes there are, he likened them to foot size and fingerprints, meaning there is an infinite number because everyone is unique.
Instead of dividing ourselves into owls and larks, he stressed, we should be speaking of an owl-lark spectrum.
Roenneberg says the best way to determine your chronotype is to identify your preferred midpoint of sleep. To do that, calculate your average sleep duration, divide the number in two, then add the outcome to your average bedtime on free days.
If you go to bed at 11 and wake up at 6, for example, add 3 1/2 hours to 11. Your midsleep is at 2:30. His research shows that 60 per cent of the population has a midsleep from 3:30 to 5am. Women tend to have earlier midpoints than men, he noted, a difference of up to two hours.
Problems arise, Roenneberg said, when there's a disconnect between our preferred sleep times and what our personal or work lives demand of us. Roenneberg calls this "social jet lag", which he defines as the difference between your midsleep on free days and on work days.
Over 40 per cent of his research subjects have social jet lag of two hours or more. In relationships, this gap can be especially pernicious, he said, as sleep schedules become a convenient scapegoat for problems that have nothing to do with sleep.
The good news is that we can adjust our internal clocks. Researchers have found that camping resets our natural sleep time to be more in line with nature. But for most of us, who work indoors under artificial light all day and stare at screens all evening, trying to adjust for the sake of our bed mates is likely to fail, Roenneberg said.
"It will be very hard to demand of your partner to override their internal clocks in order to spend more time together," he said. "It's possible, but not very beneficial, I think. If you don't sleep during your own internal timing window, you will not be as socially capable or as effective at work, and you will have somebody to blame for it, and that is your spouse."
Also, having different sleep schedules can benefit relationships, he said. Those with babies can time-shift caring for the children, and others can schedule time to themselves.
Heather Gunn is a psychologist and couples sleep researcher at the University of Pittsburgh who also advises patients in a sleep clinic.
She said that the most important thing she's learned is that couples do not need to sleep at the same time to have a healthy relationship.
"There's even some evidence that well-adjusted couples who have mismatched sleep schedules are actually much better at problem solving," she said.
She advises couples who sleep at different times to make sure they find other times to connect, whether it's the morning, the half-hour before the first partner goes to sleep, or even the weekend.
The New York Times