Every household seems to have one. While everyone else is yawning by nine, the night owl is just getting started, reading, puttering, or watching one more show long past midnight. Come morning, the same person can barely string a sentence together before coffee. Is it habit, or is something else going on?
Scientists say a lot of it comes down to your chronotype, your body's natural preference for when to sleep and when to be alert. Some people lean toward mornings, others toward evenings, and most fall somewhere in between. These preferences are partly inherited, shaped by age, and nudged by things like light and daily schedules.
Larks, Owls, and Everyone in Between
Researchers describe two extremes: morningness, for people most alert early in the day, and eveningness, for those who peak at night. The everyday names are larks and owls. In Scandinavia, people sometimes call them A-people and B-people.
In normal variation, an evening type's whole sleep and wake cycle can run about two to three hours later than a morning type's. That's a big gap if you're trying to share a breakfast table. When owl tendencies become so strong that a person can't function in normal work or school life, doctors may consider it a sleep disorder, so if that sounds familiar, talk to your doctor.
It's Partly in Your Genes
Your body runs on a circadian system, an internal clock found in cells throughout the body and brain. It helps control hormone levels, body temperature, metabolism, alertness, and sleep.
Studies have linked 22 genetic variants to chronotype, many sitting near genes involved in sensing light and keeping circadian rhythms. That makes chronotype genetically heritable. One gene variant associated with being an evening person and with daytime napping may even trace back to interbreeding between early modern humans and Neanderthals.
Genes aren't the whole story, though. For most people, chronotype likely comes from genes interacting with the environment. Light is a major cue, along with mealtimes, social life, and work or school hours.
Your Clock Shifts as You Age
If you've noticed you wake earlier than you used to, you're not imagining it. Young children tend to prefer an earlier sleep schedule. Teenagers swing the other way, preferring to stay up later and sleep in. Then, later in life, many older adults drift back toward early nights and early mornings.
Age changes what counts as late, too. The researchers behind a widely used morningness questionnaire noted that going to bed at 11:30 p.m. might mark a morning person among college students but an evening person among people aged 40 to 60.
Why Would Evolution Make Owls?
Nobody knows for sure, but some researchers have proposed a link between chronotype and nighttime vigilance in ancestral societies. The thinking goes that people who naturally stayed up later could help keep watch while others slept.
Staying up late wasn't always seen as normal. Being awake after dark was long considered a negative trait. That began to change in Europe in the 1600s and 1700s with more household lighting, street lamps, coffee, tea, and cafes that stayed open until dawn.
Today, some employers allow flexible hours to suit different body clocks, and in Denmark a group called the B-Society argues for just that. So if you're the one still awake when the house goes quiet, you're not broken. You may simply be running on a slightly different clock.
