According to the study titled The possible role of human milk nucleotides
as sleep inducers published in Nutritional Neuroscience, a link was found between certain levels of nucleotides contained in breast milk over a 24 hour period.
The conclusions portion stated the following:
The assay of nucleotides in the breast-milk of the
study population showed that their levels were not
constant over a 24-h period. This was particularly so
for 5′AMP, 5′UMP, and 5′GMP, which showed
increased concentrations at night and may, therefore,
be involved in inducing hypnotic action in the infant.
For a link to the full study, click here
I'd like to clarify some misinterpretations that seems to be occurring. The intent of me posting the paper and referencing it was to show that breast milk changes throughout the day. The paper does not prove that this induces sleep for babies but only states that it is possible (note the title of the paper). Please do not misinterpret this. OP's partner is partially right and partially unproven.
Yes, breast milk seems to follow a circadian rhythm. It is not clear if these chemical changes induce sleep in the receiving baby.