Tired and cranky: Babies who have been awake for a while are more sensitive to sad and angry faces

This post is part of our series on Infant Sleep and its Impacts on Development, published in collaboration with the journal Infant Behavior and Development. The featured research appeared in a special issue on how infant sleep affects cognitive, social, and physical development and how parents and practitioners can help promote healthy sleep and development in infancy.

Key takeaways for caregivers

  • Babies’ intake of new information that is emotionally relevant might vary depending on when they last slept.
  • After being awake for an extended period, babies become attuned to negative emotional information.
  • Supporting babies in their sleep requirements, such as by maintaining consistent bedtime rituals, can support their well-being.

Sleep helps babies grow and develop

Have you ever planned to take your baby to playgroup but decided not to because it was nearly naptime? Many parents assume that their baby’s readiness to engage with others is connected to how wide awake or sleepy they are. But does all their learning happen only when they are wide awake?

Babies were actually better at recognizing angry and sad faces after they had been awake for a longer time.

Babies spend most of their time sleeping. Rather than being a waste of time, sleep helps their growing bodies and brains: Sleep plays an important role in babies’ physical growth and cognitive development, including their learning and memory. Babies who nap soon after learning new information remember more of the newly acquired information and can use it more effectively to solve new problems than can infants who do not nap soon after learning.

Compared to these insights into the benefits of sleep when it occurs after learning, we know little about the relevance of infant sleep that occurs prior to a learning opportunity. In everyday life, parents might observe their baby getting cranky when naptime or bedtime approaches. However, researchers have not extensively studied whether babies process information differently depending on whether they have recently slept.

Does sleep enhance babies’ learning of emotional information?

Surprisingly little research has been conducted on whether infants’ learning of emotional information is affected by their sleep patterns. To start addressing this gap in knowledge, we asked: What are the effects of sleep timing on six-month-olds’ recognition of emotional faces?

tired baby sleeping

Photo: Tim Dennell. Creative Commons.

We focused on recognizing emotional faces because faces are frequently encountered and are important visual stimuli for babies. Babies learn about faces quickly. From birth, babies prefer to look at faces over other visual patterns. They quickly begin to recognize the face of their caregiver, and prefer to look at faces more like the ones in their environment (e.g., preferring faces of people of their race over faces of people of other races).

Beyond the value of recognizing familiar faces, faces are also important because they display social and emotional cues that mirror a person’s mood. Keeping in mind who looked friendly and who looked angry might be particularly important for babies, who depend on the care of others for their survival and comfort.

Studying infant sleep and recognition of emotional faces

We were interested in discovering how easily babies recognized human faces showing different emotional expressions based on whether the babies had recently slept or been awake for an extended period. Because research has shown that sleep benefits babies’ learning and memory, we predicted that babies would find it easier to keep emotional faces in mind when they were well rested than to do so when they were sleepy.

We visited 17 six-month-olds and their caregivers in their homes over two days. One day, we visited after the babies had awakened from a recent and long nap. The other day, we visited toward the end of the babies’ longest period of wakefulness (which averaged 140 minutes).

The babies in our study may have been better at recognizing sad and angry faces when they were sleepy because the negative information matched their own current emotional state.

On both occasions, we had each baby sit on their caregiver’s lap and tested infants’ visual memory through a procedure commonly used in research. Babies were shown pictures of female adult faces displaying neutral, sad, or angry expressions. We filmed babies’ looking times to each face using a hidden camera, arranging the presentation in the same way each time: First, babies saw a picture of a person (for example, looking angry). Next, they saw the same picture next to a picture of a new person with the same emotional expression as the first one.

When babies are shown a picture for a longer time, they grow tired of it (just as adults do) and pay less attention. When they see a new picture alongside the old picture, they pay more attention to the new one, but only if they remember the old one. If they do not remember the old picture, they might look at both the old and the new pictures for the same amount of time.

focused baby

Photo: Hessam Nabavi. Unsplash.

Babies had better memory for angry and sad faces after being awake

Using this logic, we found some surprising results. In contrast to our predictions that recent napping would strengthen memory, babies were actually better at recognizing angry and sad faces after they had been awake for a longer time. They failed to recognize these kinds of faces when they had recently slept. In other words, it appeared that the babies were particularly receptive to emotionally negative information after they had been awake for a long time.

How might babies see their social world at different stages in their sleep-wake patterns?

The babies in our study may have been better at recognizing sad and angry faces when they were sleepy because the negative information matched their own current emotional state. As babies get tired, they can become grumpy which, in turn, might lead them to process information that matches this state. Researchers call this mood-congruent learning. While we did not test this explanation in our study, it should be an avenue for further research.

tired and cranky

Photo: Jerald Jackson. Creative Commons.

Although our study was small, the results suggest one mechanism that might link early sleep problems and later impairments in mental well-being. Assuming that sleep problems regularly lead to fatigue and delayed sleep onset, affected babies might be susceptible to taking in emotionally negative information efficiently and storing it in their memory. As a consequence, the developing knowledge base of infants with sleep problems versus infants without sleep problems could be quite different, leading to different, perhaps more pessimistic, views on the (social) world. These speculative ideas clearly require more research.

Our results suggest that timing of sleep could influence which type of information babies focus on and process. What does this mean for parents? Due to the small size of our study, our findings about processing emotional information must be considered preliminary. However, it is clear from previous research that sleep plays an important role in early development.

Having a calm and consistent bedtime routine helps babies make the most of their learning and the fun interactions they have had during the day. Learning to read babies’ early signs of tiredness, and adjust to changing sleep schedules as they grow, can help babies enjoy the benefits of good sleep.

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