In humans, the
end product
of the HPA axis is cortisol, a steroid hormone that follows a diurnal rhythm — increasing early in the morning, peaking approximately 30 minutes after waking, and declining throughout the day, reaching
near - zero levels at night.14 This diurnal pattern is not present at birth but begins to emerge around 3 months
of age15, 16 and is fully entrained to daylight cycles by age 2
years.17 Children experiencing social deprivation or maltreatment show departures from this typical profile
of diurnal HPA
activity, suggestive
of chronic stress.