Good question and one I ask myself a lot. One thing I try to look at is whether I'm not doing an activity because I'm more excited by the prospect of down time/time at home or whether it's because I dread being around others and can't face going out. Also are there reasons why I might be particularly exhausted and need a break? Finally, when I look back on it in a day or two am I glad that I took me time or disappointed that I missed something.