A month ago, my 6MO incurred quite a bit of sleep debt due to missed naps, over-tiredness, and occasional later bedtimes (730PM). I noticed he was waking up often in the night and especially between the hours of 4AM to 5AM. As a result, he was not well rested and had to catch up on his sleep in the day. His morning wake time was extremely short and he was basically using the morning as an extension of his night sleep. This disrupted his circadian rhythm quite a bit and during times where he should be energetic and active, he was listless and tired (yawned quite often).
NOTE: Wake time for a 6MO is approximately 2 hours (can be 1.5h in the morning to max 2.5h before bedtime).
I read books by Weissbluth, Ferber, and several online blogs by experienced mommies. Initially I took Ferber's advice by limiting his sleep hours and pushing bedtime later, boy this was a HUGE DISASTER.
I recalled what my sleep consultant mentioned, "Sleep begets sleep" and this was also mentioned by Weissbluth and several other sleep experts. I plucked up the courage to push his bedtime early (sleep by 5.30PM), once even as early as 4.30PM, and over a period of one and a half weeks, he managed to recover his sleep debt.
On days where naps are terrible (less than the desired amount) or missed, a 5.30PM bedtime is recommended or depending on your family's schedule, as early as 1h to 1.5h before typical bedtime. Many parents think that a later bedtime is better for their babies as it pushes their wake up time. NOT TRUE (unless your baby naps MUCH MORE than the desired amount in the day)!
Babies' bedtime should come in approximately between 12 to 13 hours after their morning awake timing. For example, a 6MO wakes at 8AM. She should be asleep by 9PM to have at least 11 hours of sleep.
An earlier bedtime does not mean that babies will wake up earlier. In fact, an earlier bedtime allows their cortisol (stress hormone) levels to drop further such that between 4AM-5AM, the cortisol:melatonin ratio will be lowered to extend sleep (easier to fall back to sleep despite being in the light sleep zone) in contrast to having a situation where your baby finds it hard to get back to sleep and is up crying.
It's like us adults, waking up at the wrong time and taking forever to enter snooze land. Babies are more sensitive (and impatient) than us hence they will cry out more during prolonged and frustrated night awakenings.