I know this is an unsatisfying answer, but as far as I can find, no one really knows for sure what causes nighttime crying spells.
This tendency of small babies to cry for extended periods at the same time every evening can be termed "colic", "purple crying" or "the witching hour", and is extremely common, but doesn't have a proven cause.
Some theories that have been set forth by various mothers, doctors, experts and bloggers are:
- The baby is overstimulated from the accumulated learning experiences and sensory input of the day
- Baby's evening cry stimulates mothers to feed the baby more frequently or longer in the evening, preparing the baby to sleep and go without feeding for longer, or to store up energy for growth, which mostly occurs overnight.
- There is a dip in prolactin hormone in the evening, so nursing mothers tend to produce less milk than during other times of day. The crying may cause a spike in hormones that stimulate more milk production to compensate.
- It's somehow related to circadian rhythms and the daily hormonal/chemical cycle
- It is a side effect or result of the baby's brain or behavioral development
- It is a side effect of the baby's digestive system and related hormonal signaling developing
- It's a residual survival instinct, as an evening-fussing baby would be less likely to be left unattended at the time of day when predators are most active
- Nighttime is too quiet and the baby feels insecure without the constant 'white noise' of the womb
- It is a symptom of an allergy, intolerance or sensitivity to something in the formula or the mother's diet
- This one sounds the strangest but actually has the most (although still not much) science behind it: To prevent mom from getting pregnant again!
As far as I am concerned, some of these are more convincing than others, but none of them have been convincingly proven. I personally lean towards the theory that it's probably an instinct or a normal developmental stage, because it is extremely common, starts and ends at predictable ages, and isn't usually solvable by changes in food, environment, daytime stimulation, noise, light, etc (although some babies do respond well to one or more of these changes).