Difficulty Falling Asleep in Infants

Difficulty Falling Asleep in Infants: Overtiredness and "sleep windows"

The situation where a tired child, instead of falling asleep, starts crying, arching, and protesting any attempt to be put down, is familiar to many parents. It seems paradoxical: why does a baby who clearly wants to sleep fight sleep so actively? In most cases the cause lies in the physiology of the infant’s nervous system, specifically a state of overtiredness. Understanding the concept of "sleep windows" and signs of fatigue is the key to easy and calm sleep onset.

The Science of Sleep: Homeostatic Pressure and Circadian Rhythms

Sleep onset is regulated by two main processes in the body. Homeostatic pressure, or "sleep pressure," is the need for sleep that builds up with each minute of wakefulness. The longer the child stays awake, the stronger this pressure becomes. Circadian rhythms are our internal biological clock that regulates the production of sleep hormones (melatonin) and alertness hormones (cortisol). For easy sleep onset, sleep pressure needs to be high while cortisol levels are low.

"Over‑regulated": Why Overtiredness Is the Enemy of Sleep

If the child does not fall asleep in time, when sleep pressure has reached an optimal level, the body interprets this as a stressful situation. In response to stress, the adrenal glands release the hormone cortisol into the bloodstream. Cortisol has a stimulating effect; it gives the child a "second wind" to cope with fatigue. As a result, the nervous system becomes over‑stimulated, and the child finds it extremely difficult to relax and fall asleep, even with high sleep pressure. This is the state of "overtiredness".

"Sleep Windows" and Signs of Fatigue

A "sleep window" is the optimal time interval during which an infant can stay awake without risking overtiredness. These windows are very short and lengthen with age. If the child is put down within this window, falling asleep will be easy. If the window is missed, the baby "over‑regulates," and bedtime turns into a battle. That’s why it is so important to create a gentle routine and learn to recognize the early signs of fatigue (yawning, red eyes, loss of interest in play) before waiting for crying, which already indicates overtiredness. More details can be found in the article about establishing bedtime rituals.