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?

In this MomJunction post, we help you understand the reasons behind a baby smiling in sleep.

Why Does A Baby Smile During Sleep?

Babies smile, and sometimes even laugh, in their sleep because of specific brain functions induced by REM 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). As the names suggest, there is rapid movement of eyes underneath the eyelid in REM sleep and no movement in NREM.

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

  • NREM 1: The baby starts to feel drowsy and will probably start 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.

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

After the REM stage, the body returns to stage two of NREM sleep, then progresses to stage three and so on to eventually repeat the REM stage. Thus, sleep occurs in cycles with REM occurring episodically between the phases of NREM sleep.

REM, dreams, and smiling:

The eyes move rapidly during the REM stage when the brain actively dreams. 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. 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.


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