It should also be noted that there is melatonin detectable in human milk,
but only during the night where its concentration was 280 +/- 34 pmol/L in
serum and 99 +/- 26 pmol/L in milk. The authors conjectured thatt he
presence of the rhythm in milk suggests that melatonin fluctuations in milk
might communicate time of day information to breast-fed infants.  I would
agree that one should therefore start with small doses and carefully
observe the child, especially for sleep/wake disturbances.  I would guess
that it would be best to avoid taking it early in the day, as well.  Since
most poeple take it for the somnolent effect, that should not be a major
problem.

(Illnerova H; Buresova M; Presl J.  Melatonin rhythm in human milk. Journal
of Clinical Endocrinology and Metabolism, 1993 Sep, 77(3):838-41.)