Melatonin is secreted in human mother's milk with a daily rhythm - high at night, undetectable during the day (see the figure under the fold):
It has been known for a long time that mother's melatonin entrains the circadian rhythms in the SCN (suprachiasmatic nucleus) of the embryos - thus they are born with a correct phase (time of day). However, a study in rats suggests that melatonin in mother's milk is unlikely to be able to entrain the pups circadian rhythms after they are born.
So, the appearance of melatonin in the milk of breastfeeding humans may either be:
a) just a by-product of melatonin being produced for all its normal functions in the mother and really has no role for the infant,
b) it may have a function in boosting the immune response,
c) it may be directly helping the infant scavenge free radicals, or
d) unlike in rats, milk melatonin may be involved in entrainment of human infants.
Much more research needs to be done....
You can read more about biological, medical, cultural and political aspects of breastfeeding here, here, here and here.
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Empirical, albeit anecdotal. As the father of two breastfed pups, both requied months before the circadian rhythms were anything resembling ours. They kept hours more like a college senior -- and spit up about as often, too.
Yup, look at this.
More "anecdata"--both of my (breastfed) kids had day and night more or less distinguished by the first night after we brought each home from the hospital. The only thing I can think of that we did differently than most is to co-sleep (baby would nurse every few hours while going through light sleep and then fall back into deeper sleep, without waking), but I know other parents who co-slept without the same happy effect.