There was no objective and constant sun in the Underworld, so morning was whenever he woke up, and he didn't care if it was night in the mortal realm when he woke. It wasn't like it mattered, or anyone other than Hermes noticed.
Who said that a day needed to have the same number of hours as every other day anyway? It was ridiculous.
The dead would have to deal with his constantly changing schedule. They had no reason to complain anyway, most didn't even sleep. Besides, they were the dead ones and were allowed to look it. He still needed to look alive.
The only person allowed to complain was probably Persephone and she didn't mind it all too much, or he was sure he would have heard of it if she did. He thought she rather liked the possibility of being able to determine that so that we were always on time.
Who liked waking up feeling like they had never slept at all anyway? Imagine shouting at someone and having to yawn halfway through?
The effect of the shouting would be completely ruined.
Mornings start only when sleep ends, after all.
Anything different, was simply not very well thought out.
AN: 100 Prompts/100 Drabbles Challenge [Morning]