Why Do Babies Smile In Their Sleep?

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There is nothing more heartwarming than a baby’s smile. As a new parent, you excitedly wait for your baby to smile. And one day, you are pleasantly surprised when you see the baby smile in their sleep.

Your immediate reaction could be immense love and adoration for the baby, and perhaps a little amusement. But then, you may get curious about it. What made the baby smile in their sleep? Does it mean something?

We help you understand the reasons behind a baby smiling in sleep.

Why Does A Baby Smile During Sleep?

Unlike the “smiling response,” which occurs later in infancy, there is a form of smiling known as “endogenous smile” occurring in newborns and is not the result of external stimulation. Babies smile, and sometimes even laugh, in their sleep because of specific brain functions, as a manifestation of endogenously determined physiological rhythms during the rapid eye movement (REM) state sleep, the stage in which we dream (1). The baby’s smile is a response to the dream. It is nearly impossible to tell the contents of the dream since babies cannot express their thoughts. But the smile is quite likely a response to an incident in a dream.

Infants use a smile as a communication tool. So a baby may smile if they dream of a parent or guardian, a favorite toy or pet and smile in response to the good feelings the vision creates.

Babies may also smile just as they are falling asleep or waking up, which could be a reaction to the pleasurable or good feelings they experience at the time.

How Does Sleep Cause A Baby To Smile?

To know why babies smile in their sleep you need to understand sleep and its stages. The two fundamental phases of sleep are non-rapid eye movement (NREM) and rapid eye movement (REM).

Babies begin their sleep with the NREM phase, which has four stages (2):

  • NREM 1: The baby starts to feel drowsy, eyes droop, may open and close dozing.
  • NREM 2: This is when the baby is slightly asleep and will awaken if there are sounds and other disturbances around.
  • NREM 3: It is the deep sleep stage where the baby is quiet and does not move.
  • NREM 4: It is the deepest of NREM sleep, and the baby continues to stay still unless there’s a loud noise to wake them up.

A baby enters stage 1 of the sleep cycle, and moves into the subsequent stages 2, 3, 4, before coming back to 3, 2, and REM. REM occurs between the above stages of NREM sleep in the following manner:

NREM 1 → NREM 2 → NREM 3 → NREM 4 → NREM 3 → NREM 2 → REM → NREM 2

Thus, sleep occurs in cycles with REM occurring episodically between the phases of NREM sleep.

REM, Dreams, And Smiling

REM (rapid eye movement sleep). This is a light sleep when dreams occur and the eyes move rapidly back and forth. Sleep experts believe that the eye movements are somewhat related to the dreams being processed by the brain (3).

The REM stage causes an increase in brain activity, sometimes to an extent to wake the person up (4). The electrical patterns of the brain during REM sleep are similar to those that are active when we are awake. This suggests that dreams during REM sleep occur because the brain seems to simulate reality.

Babies tend to sleep for 16 hours a day, half of which is REM sleep. Older children and adults sleep fewer hours and spend much less time in REM sleep. Therefore, it is not surprising to see the baby smile while they’re asleep during the day or night. The next time your baby smiles during sleep, it means they are sleeping well and having sweet dreams too!

Want to share your experience of seeing your baby smile in sleep? Do tell us in the comment section below.


MomJunction's articles are written after analyzing the research works of expert authors and institutions. Our references consist of resources established by authorities in their respective fields. You can learn more about the authenticity of the information we present in our editorial policy.
1. The secrets of infants smile; Penn State University
2. Newborn-Sleep Patterns; Stanford Medicine
3. Natural Patterns of Sleep; Harvard Medical School
4. The Characteristics of Sleep; Harvard Medical School