Babies, just like adults function best when we are in alignment with our natural circadian rhyme i.e. body clock, which is in tune with the light and the darkness of a 24 hour cycle of a day.
When a child is up past their natural inclination to sleep, between 6pm and 9pm pending age of the child, his/her body will begin to secrete hormones such as cortisol. Cortisol is a stress related hormone that will stimulate the baby creating the effect of a "second wind" in order to keep them awake. As a result, the child will become more agitated, difficult to console and will need more from his/her parents to fall asleep later.
Counter intuitively, over-tired babies often become "early risers," waking much earlier then and not napping as well as rested babies. It's best to find your child's natural "sleep window." This is when their body is naturally producing melatonin, a hormone "that promotes sleep by relaxing our muscles and making us drowsy" (West, 2009). Establishing healthy sleep patterns in relationship to your babies biological rhythm will contribute to your babies healthy development, allowing them to sleep through the night and nap well, which also means fewer sleep deprived parents.
Reference: West, K. (2009). Goodnight Sleep Tight.
