Why Do Newborns Smile In Their Sleep?

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Sleeping is a very complex process. During sleep, the brain is essentially at rest, and the body creates an active state of unconsciousness that is mostly responsive to internal stimuli. [1]

Newborns smiling in their sleep is natural. According to a study, two-month-old newborns exhibit smiling behaviours during active and waking sleep. [2]

Some studies say that a baby smiling in its sleep is a sign of good emotions like joy, while others say that it is just a reflex rather than a real expression of happiness. So, what exactly does it mean for a baby to smile in sleep?

Sleep Cycle

There are two types of sleep cycles: REM (rapid eye movement) and NREM (non-rapid eye movement), which further have three stages, N1-N3. The body goes through each stage four to six times during the course of one night, with each cycle lasting an average of 90 minutes. [3]

The sleep cycle starts with NREM Stage 1, followed by NREM 2 and 3, before moving to the REM cycle. During NREM Stage 1, the body starts relaxing and breathing gets slower. It is the stage when a person goes from being awake to being sleepy.

At stage 2, the breathing and pulse rate continue to calm down and the muscles continue to relax, followed by a decrease in brain activity and the cessation of eyelid movements. NREM Stage 3 is the last stage, and in this stage, breathing, muscle movement and brain waves are all at their lowest points.

After the third stage of the NREM cycle comes the REM stage. It takes around one and a half hours to come to this stage of sleep when most of the dreaming takes place. At this stage, the movement of the eyelids starts, brain and heartbeat increase, but there's a paralysis of the body parts of the torso to prevent movement while dreaming. Studies say that most episodes of smiling mainly occur during the REM cycle. [4]

Sleep Cycle In Newborns

According to a study, newborns and infants (a year old and younger) have undistributed sleep timing of sleeping and waking as they sleep for around 16-18 hours a day.

Newborns have different types of sleep cycles known as active sleep (like REM), quiet sleep (like NREM) and indeterminate sleep. Compared to children and adolescence, newborns' sleep cycles are likely to start from the REM cycle or active sleep instead of NREM, probably due to their underdeveloped circadian rhythms.

At around two or three months, the circadian rhythms, also known as the sleep-wake cycle, start developing, causing the REM cycle to decrease and the NREM cycle to increase.

As newborns experience more REM sleep, the possibility of smiling is higher as a result of reflex or involuntary movements, than the actual 'dream'. In newborns with epilepsy, seizures may cause uncontrollable giggling.

Other reasons include the evolution of emotions in newborns and the passing of gas.

To Conclude

Seeing a newborn smile while sleeping is an important indicator of how emotionally and physically the infant is progressing. Every baby has different developmental milestones. Therefore, whether your newborn smiles in the sleep a lot or not shouldn't be taken as a serious growth factor.