I never actually believed it when I learned that it takes more muscles to frown than it does to smile? It's true. 

So now, I wonder, why it is after a good set of tears, one could feel so exhausted? Why does it take so much energy to expel such sadness? It's not a good exhausted, either. It's more of a 'take me to bed, throw the covers over me and let me sleep this off exhausted'. Of course, there is also the exhaustion that comes after excitement but that one always leaves (me at least) wanting more. 

I suppose it's simple. There are types of exhaustion. 

I, my friend, am exhausted. Bed time, under the covers, let me sleep it off - exhausted.