Discussing the effects of daycare on children continues to be a controversial undertaking. Until now, research has focused on later outcomes for children, such as language skills or "school readiness" at the age of five or six.
But what has proved elusive is an understanding of how a young child is affected emotionally and physiologically, and how they experience daycare while they are actually there. Babies cannot speak and toddlers have limited verbal abilities when it comes to describing their inner world.
However, a new generation of research from the biosciences is starting to provide an illuminating glimpse into this unspoken landscape, with children's neuroendocrine responses helping to shed light on their experience. And some of those responses paint a worrying picture: there is something about attending daycare for an extended time - whether small-scale and home-based or large-scale and centre-based - that often triggers stress and neuroendocrine changes in young children that could have potential long-term consequences for their mental and physical health as adults.
The College of Education and Human Development at Minnesota University reports that, in many cases, 70-80 per cent of children in centre-based daycare show increasing levels of the stress hormone cortisol throughout the day, with the biggest increases occurring in toddlers. Yet by the age of five, children no longer exhibit these stress reactions.
A meta-analysis of nine studies - Children's Elevated Cortisol Levels at Daycare - concludes: "Our main finding was that at daycare children display higher cortisol levels compared to the home setting. Diurnal patterns revealed significant increases from morning to afternoon, but at daycare only ... We examined all papers on possible associations between cortisol levels and quality of care, and the influences of age, gender and children's temperament. Age appeared to be the most significant moderator of this relation. The effect of daycare attendance on cortisol excretion was especially notable in children younger than 36 months."
Children's immune systems may also be affected. A recent study at Cornell University reports that "elevated cortisol in children during childcare may be related to both lowered antibody levels and greater illness frequency".
Stress may activate immune cells in a child's skin, causing eczema. A German birth cohort study this year found that, of 11 possible risk factors during the first two years of life, only "daycare centre attendance is associated with an increased prevalence and incidence of eczema" years later.
Cortisol is produced by the hypothalamic-pituitary-adrenal axis (HPAA), the system involved in a child's response to fear or uncertainty. Cortisol levels undergo a daily cycle; they peak in the early morning and decline throughout the day. Infants are born without a diurnal cortisol rhythm - they acquire it during their first year of life and it is subject to programming. The HPAA may be programmed by early childcare experiences.
Rising cortisol levels are healthy and necessary as an appropriate stress response. However, stress occurring for extended periods of time during sensitive phases of early development might cause lasting changes in the settings and function of the child's HPAA. A study by the US National Institute of Child Health and Human Development reported that children who, during their first three years, "spent more time in centre-based childcare - whether of high or low quality - were more likely to have the atypical pattern of lower levels of cortisol just after awakening when they were 15 years of age, which could indicate higher levels of early stress".
Even subtle abnormalities of the HPAA have now been implicated in stress-related disorders, including depression and anxiety. Cortisol levels at age 17 have recently been used to predict the development of psychiatric disorders during the following two-and-a-half years. Interestingly, a recent study by the US Department of Health found that both quality and quantity of daycare were linked to later adolescent functioning: "More hours of non-relative care predicted greater risk taking and impulsivity at age 15."
Others argue that intermittent separation of an infant from its mother actually promotes greater resilience and better brain function. But this research is based on free-living foraging monkeys that are biologically independent and only then separated from their mothers for just an hour a week over 10 weeks - not exactly daycare.
To better understand the well-being of children requires a more fully informed discussion. The research above is not conclusive and may not be comfortable, but we have a duty to our children not to ignore it. And it may also help explain the behaviour of pupils who enter your classroom.
Dr Aric Sigman is a fellow of the Society of Biology, fellow of the Royal Society of Medicine, and associate fellow of the British Psychological Society. Full references for all the studies mentioned in this piece can be found in his article "Mother Superior? The biological effects of daycare", The Biologist (September 2011).