Everyone reacts differently.
Indica strains are more of a sedative, never smoke these during the day or when you have some shit to do because you will be a sluggish motherhuncher.
Sativa strains are more cerebral, energizing and euphoric, smoke these whenever, except maybe right before bed.
Also, dirt weed is filled with pesticides, fertilizers, etc that cause the body to feel "burnt out". Smoke when you are finished with your business for the day. And like Kingjohn said maybe your smoking a bit too much. Get on a smoking regiment, for example I'll smoke in the morning once during the week, once on friday night and once on Sunday evening. My body knows when the herb is coming. peace