What mechanism suppresses melatonin release in humans?
Take a look at this (especially page 938): Coordination of circadian timing in mammals by Steven M. Reppert &amp; David R. Weaver Nature 418, 935-941 (29 August 2002) | doi:10.1038/nature00965


  Sodium-dependent action potentials provide the primary means by which
  the SCN transmit circadian outputs to other brain areas and are
  essential to its role as a pacemaker1.
  
  Signalling molecules from SCN efferents include neurotransmit- ters
  and secreted factors. An output role for secreted factors comes from
  the results of SCN transplant studies that suggest that the
  alternating activity of SCN-derived ‘inhibitory’ and ‘activating’
  factors drives locomotor activity rhythms (and hence rest/activity and
  sleep/wake cycles) in rodents


There was also a mention of per/cry gene expression.