Biologically speaking, what is happening when it's: summer, 21 degrees and humans feel too hot, then winter, 21 degrees in a heated room, but humans feel cold? Is it all in their heads or is there a biological reason for this?

Biologically speaking, what is happening when it's: summer, 21 degrees and humans feel too hot, then winter, 21 degrees in a heated room, but humans feel cold? Is it all in their heads or is there a biological reason for this?