Spring Mystery

How do bulbs know when it is time to pop up out
of the ground  I cover my gardens with a grass/
leaf mulch each autumn so they are well buried.
No light can reach under the leaf mulch; never-the-
less these bulbs are in a hurry.  Yesterday, I glanced
out and saw two leaf domes..."Could that be the
daffodils, I wondered?"  Sure enough, there was
at least four inches of growth under these leaf
domes.

How do daffodils know when it is their time?  Any ideas?