Betta question: sick or lazy?

I have a male betta that spends most his time at the bottom of his bowl. It's not heated so could it be too cold? If so what are some tips to heat the water(i live in a snowy state w/ not too much sun) Or his he sick?...he hasn't lost color and his fins seems to be okay...he also eats aggressively when he finally notices i've fed him.