I wonder if its more of a mental thing; you mentally associate showering with being tired. It makes sense, because most people (myself included) shower before bed so it becomes more of a ritual and the body sometimes has a hard time breaking from the ritual.
Maybe try and break the cycle by maybe getting really active after a shower. Might seem silly, but after a shower in the morning I used to turn on dancing music and dance around my living area as I am getting ready.
Just a thought 
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"Don't be a Dork! Wrap your Pork!"
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