“Daycare or preschool stress can be measured by the levels of cortisol-—a stress hormone—-that children produce during the day. In normal, healthy people, cortisol levels follow a daily rhythm, peaking when they wake and then falling over the course of the day. Cortisol levels are the lowest just before sleep (Sapolsky 2004). But stress changes the pattern. If you are under stress, your cortisol level rises, regardless of the time of day. In the short term, this helps your body respond to the crisis. But chronic stress, and chronically elevated levels of cortisol, can cause health and developmental problems (Sapolsky 2004). Because cortisol levels are easy to measure in young children, researchers have collected samples from children who attend daycare and children who stay home. In study after study, the results are the same. When children stay home, their cortisol levels show the healthy pattern–rising at waking and decreasing throughout the day. When children attend daycare, the pattern  changes. Cortisol levels increase during the day (Geoffroy et al 2006). See more at: http://www.parentingscience.com/preschool-stress.html#sthash.DxbP97o1.dpuf