You're tired. Mentally drained. Low on motivation to do anything but sit down and switch off in front of the TV..... STOP!!!
As soon as you start exercising you will feel better. Don't think of exercise as something that tires you out. Yes it uses energy but it increases your circulation, gets more oxygen around your body, helps remove waste products in your body, and releases endorphin's that will make you feel awesome.
In a world where realistically we don't need to be fit spending time exercising can be a hard thing to rationalise... but people that get addicted to exercise aren't crazy (all the time). You are going to get more out of it than you put in. Just do it and break the cycle. Before you know it doing some exercise will be one of the highlights of your day.