http://news.bbc.co.uk/hi/english/health/newsid_401000/401991.stm
"In the morning, they found, two proteins combine to produce a third
chemical which, after about 12 hours, has increased sufficiently in volume
to trigger another chemical reaction - which stops production and sends the
body to sleep. .... The cryptochrome, which is light sensitive, is the
trigger mechanism. "
