A leading researcher on temperament in infants and young children once remarked, “When I raised my first child, I believed behavioral theories claiming that what I do as a parent molds my child’s character. With my second child, I was already a geneticist and believed that a child is born with characteristics passed on through heredity, with minimal environmental influence. By the time I had my third child, I barely knew them at all.”

This exaggerated analysis highlights the ongoing quest of parents and scientists to answer a crucial question: what determines the personality and characteristics of a child? The debate between heredity (“She got her shyness from her dad’s family”) and environment (“If his mother were stricter, he would be calmer”) underlies parents’ attempts to understand their influence on their child’s development.

The Influence of Heredity and Environment:

Current research paints a complex picture, showing that both heredity and environment significantly influence a child. Evidence suggests that a baby is born with genetic traits that determine not only physical characteristics but also personality traits such as activity level, shyness, sociability, openness to new situations, and anxiety. Many parents find that their child has traits they dislike, especially if they remind them of qualities they see in their parents, spouses, or themselves. Trying to change these traits often proves to be a losing battle.

The most critical variable influencing the parent-child relationship is the “goodness of fit” between the child’s traits and the parents’ expectations. For example, a very active child may be adored by a father who appreciates and identifies with this trait but merely tolerated by a father who expects a calmer child. Conversely, a quiet, calm child might be seen as perfect by one father but considered depressive or lifeless by another. Incompatibility between parental expectations and the child’s traits can lead to frustration and stress, particularly if parents try to “correct” the child.

Temperament and Sleep:

Every parent knows the signs of a child “up past their bedtime.” Some children calm down and fall asleep on their own, while others become restless, irritable, and uncooperative. Fatigue doesn’t always result in obvious sleepiness; sometimes, it leads to hyperactivity and negative behaviors that resemble behavior disorders.

Studies show a strong correlation between sleep and the development of a child’s personality traits. Babies with sleep disorders, such as difficulty falling asleep or frequent awakenings, tend to be more demanding and sensitive to stimuli. In one study, mothers of babies with sleep problems described them as more difficult in various behavioral domains compared to babies without sleep problems.

One particular trait measured is sensitivity to sensory stimuli (noise, temperature, taste, smell). Babies who are hypersensitive to sensory stimuli often develop sleep difficulties because they struggle to disassociate from environmental and internal signals. This hypersensitivity interferes with their ability to relax, fall asleep, and stay asleep.

Sleep and Behavior in Older Children:

The correlation between sleep and behavior continues as children grow. Studies have found that sleep disorders in school-aged children are linked to behavior problems and difficulties in adaptation. Sleep disorders are prominent signs of stress, anxiety, depression, and other adaptation issues. For example, sleep problems are often included in the diagnostic criteria for anxiety disorders.

Sleep disruptions can lead to behaviors that resemble hyperactivity. Parents often describe their restless babies as having a “turbo engine” they can’t shut down at bedtime. There is evidence that sleep disruption can cause hyperactive behavior patterns, even if no direct research confirms this. Lack of sleep can lead to fidgeting and increased activity as methods to stay awake, contradicting the expectation that tired children will relax.

Intellectual Development:

Assessing intelligence in infancy is complex. Tests for early mental abilities often fail to predict future intelligence or cognitive abilities. Research on sleep and intellectual development is limited by this challenge. However, studies on older children and adults show that sleep disorders primarily interfere with cognitive abilities related to attention and concentration. Insufficient sleep results in slower reactions and more mistakes on tasks requiring continuous attention.

Indirect evidence from mothers’ descriptions of their babies supports the correlation between sleep and attention. Babies with sleep problems often have trouble concentrating on activities and are easily distracted. Studies on school-aged children using sleep watches and computerized tests found that poor sleep quality (frequent awakenings) led to decreased attention abilities.

In conclusion, while direct research on infants is limited, evidence from older children and adults suggests that disrupted or insufficient sleep can challenge intellectual abilities. Ensuring good sleep habits early on is crucial for a child’s overall development and well-being.