Do you ever wonder why your baby can't understand the difference between day and night?

As adults we usually sleep according to our circadian rhythm - this is basically the natural patterns that happen every 24 hours which are controlled by the biological clock in the brain which very cleverly syncs to your external environment. Babies aren't born with this circadian rhythm so they don't know to go to sleep because it is dark like we do. This develops over time as exposure to light and dark in their environment helps newborns develop a sleep-wake cycle that coordinates with periods of day and night. Two hormones that affect when newborns sleep and when they wake up are melatonin and cortisol. As infants reach 8 to 9 weeks old, the release of these hormones begins to follow a circadian rhythm. This is the period when an infant's sleep may start to become more predictable. This is why experts recommend 4-6 months of age as a time to start sleep training!