Ever feel that you're decaying mentally?

Lately my attention span has been decreasing. I can hardly watch a movie without zoning out. My cognitive functions are slower than they used to be, things make less sense to me, reading has become a little more than a chore and I often find myself looking for things that will talk to me or show me instead of reading a “how-to”.

Does anyone have an idea on how to get my mental edge back?

Side details:
I’m mid 20’s, I exercise regularly, college graduate. Winter hit and I no longer have hobbies since they are all warm weather activities. I live practically alone since my roommate is always with his girlfriend. I don’t have many friends; I work IT so I’m constantly staring at a computer screen and I work alone.