The shutting down of the brain is it forcing your body to accept that it's time to go to sleep. Typically, the shutting down happens simultaneously to falling asleep...or for night owls, about 4 in the morning when you realize you have the imprint of the keyboard on your forehead and all you've said is a slurry of nonsensical alpha-numerics.

You could simply take the que and go to bed when that starts happening...or go with Mandymae's advice and half the dose and take the other half right before you go to sleep. Don't take them too far apart, though or you won't have the same full effects. I'd say no more than 45 min apart.

Also, the panicking isn't uncommon if you're not used to the feeling of your brain chemicals forcing your body into sleep. Take a tip from Douglas Adams and "The Hitchhiker's Guide to the Universe" :: ~DON'T PANIC~ ;p