Why Do Babies Smile in Their Sleep?

Of course, as many parents will attest to, babies may also show a reflexive smile if they are passing gas or stool. (Possibly out of relief!)

What Happens to My Baby While They Sleep Anyway?

Like adults, newborns spend time in REM (Rapid Eye Movement) sleep—just a lot more of it. 

“Newborns will spend up to half their sleep in REM, while adults will typically spend 20 percent of the night in REM sleep,” explains Daniel Combs, M.D., board-certified in pediatrics and sleep medicine and an assistant professor of pediatrics and medicine at the University of Arizona. “Adults will typically have their first REM sleep about 90 minutes after they fall asleep. Infants will typically go straight into REM sleep.”1

That sweet smile you notice during nap or bedtime is likely occurring during this period of REM sleep. Dr. Combs explains that you can actually tell when your little one enters REM sleep, as they will often show frequent eye movements and twitches of their arms and legs.

“Newborn sleep is classified as quiet or active sleep, which corresponds to non-REM or REM sleep in adults,” he adds. “By age 1, babies will no longer go directly into REM sleep. By age 5 or so, most children will only have 20 percent REM sleep.”

Additionally, your infant's circadian rhythm (or internal body clock) is not in sync with the 24-hour day and night cycle that adults are used to, leading to erratic sleep patterns. “Most newborns can sleep in one to three-hour sleep cycles throughout the day and night. As their bodies grow and feeding needs change, those cycles can lengthen, and more sleep will occur at night,” says Dr. Priest.

How Much Sleep Does a Newborn Need?