sleep science for babies and children: understanding circadian rhythms, the wake maintenance zone and key hormones
Sleep is a fundamental biological process essential for the growth and development of babies and children. However, its mechanisms are complex and involve various biological systems. In this article, we'll explore the science behind sleep in young children, including the circadian rhythm, the wake maintenance zone, homeostatic sleep pressure and the roles of melatonin, cortisol and the stress response.
The circadian rhythm: our internal clock
The circadian rhythm is a natural, 24-hour cycle that regulates sleep and wakefulness. In newborns, this system is immature and takes time to develop. By around three to four months of age, babies begin establishing a circadian rhythm influenced by external cues such as light and darkness.
Factors influencing circadian rhythm in children include:
Age and development: Newborns have erratic sleep cycles, but by six months, a more structured rhythm emerges.
Light exposure: Artificial light, especially blue light from screens, can disrupt the natural cycles.
Behavioural patterns: Irregular sleep schedules can misalign the circadian rhythm, leading to sleep disorders like insomnia or jet lag.
Activity levels: Physical exercise can act as a strong signal to the body's internal clock, helping to synchronise and regulate the sleep-wake cycle.
Social cues: Meal times, social interactions and daytime schedules can all impact sleep-wake patterns.
Eating: Consistent mealtimes help to synchronise the internal clock, while irregular mealtimes can disrupt this rhythm – leading to potential health problems.
Habits: Irregular sleep patterns, meal times and exposure to light at specific times of day can all affect the circadian rhythm and impact sleep quality and overall health.
Hormones: Melatonin and cortisol increase or decrease in response to light and dark cycles. These hormonal changes help regulate sleep, alertness and other behaviours.
Homeostasis: Acts as "sleep pressure" that builds up during wakefulness, prompting the body to seek sleep at the appropriate time depending on the circadian clock.
The wake maintenance zone: fighting sleep
The wake maintenance zone (WMZ) occurs in the evening when a child's body naturally resists sleep. This period can lead to increased activity and alertness, making bedtime a challenge. This resistance is due to a combination of natural wake-promoting hormones, such as cortisol and dopamine.
To help children settle during this phase:
Keep bedtime routines calm and consistent
Reduce stimulating activities before sleep
Dim lights in the evening to signal the body that sleep is approaching
Homeostatic sleep pressure: the need for sleep
Homeostatic sleep pressure refers to the accumulation of the need for sleep throughout the day. Young children require more sleep than adults, and their bodies build sleep pressure more quickly.
Newborns sleep in short cycles due to their immature sleep systems.
Toddlers and pre-schoolers still require naps to balance sleep pressure and avoid overtiredness.
School-age children can benefit from early bedtimes to replenish sleep needs.
Adenosine, a chemical that promotes sleepiness, builds up in the brain over the day. If children miss their ideal sleep window, they may become overtired – making it harder for them to fall asleep.
Melatonin: the sleepy hormone
This hormone is secreted by the pineal gland in the brain. While it doesn’t induce sleep, it does make us feel sleepy and improves our quality of sleep. It begins excreting in the presence of low light levels and is inhibited by bright light and artificial blue light. Melatonin isn’t endogenously produced by infants at birth and it may not begin to be secreted until round 12 weeks of age. However, the average time seems to be around 1-2 months.
Factors that affect melatonin production in children:
Light exposure: Evening light, especially from screens, can delay melatonin release.
Consistent sleep timing: Regular bedtimes help synchronize melatonin release with sleep onset.
Parental contact: Breastfeeding and skin-to-skin contact may help regulate melatonin levels in infants.
Cortisol and the stress response: the wakefulness hormone
This hormone plays a critical role in wakefulness and the stress response. It follows a daily rhythm, peaking in the morning to promote alertness and gradually declining in the evening to allow sleep.
For babies and young children, excessive stress or overstimulation can elevate cortisol levels – making it difficult to fall asleep. Separation anxiety, overtiredness and inconsistent routines can all contribute to higher cortisol levels before bed.
To support healthy cortisol regulation:
Provide comfort and reassurance during bedtime routines
Create a soothing sleep environment with dim lighting and white noise
Encourage calming activities before bed, such as reading or listening to lullabies.
Final thoughts
Understanding the biology of sleep in babies and children can help parents foster healthy sleep habits. By aligning sleep schedules with natural circadian rhythms, recognising wake maintenance zones, balancing homeostatic sleep pressure and managing sleep-related hormones, caregivers can improve sleep quality and promote overall wellbeing. Establishing consistent routines, reducing screen exposure before bedtime, and creating a calm sleep environment are key steps in supporting a child's healthy sleep cycle.