Actually dark period does. Not getting very detailed but at night your plants develop hormones with the proper dark period they will develop enough hormones to flower.
Not sure if there actually called "hormones" but you get the picture, i hope
MH is the best for veg but what is most important is the spectrum for growth.
During the Vegetative state plants need more Blue spectrum hence Metal Halides. Blue Spectrum CFL's and Fluoros would do the trick too but MH is best.
During the Flowering state plants need more Red spectrum hence High Pressure Sodium. Red Spectrum CFL's and Fluoros would do the trick too but MH is best.
Now MH and HPS and HID (High Intensity Discharge) so you will to deal with the heat, this is the more expensive but more rewarding option.
CFL (Compact Fluorescent Lighting) using the designated color are also reasonable if you would like to keep cost/heat down. Used for smaller personal operations.
Also MH along with HPS during flowering is thought to produce more resin, many old timers live by that.