Ok, that's a good sign actually that you feel better at night. It means your coping mechanisms must be pretty good, otherwise a bad start would make you spiral downwards further
There's a chance that at night, all this subconscious stress is being processed and relived by the mind, maybe in dreams that aren't remembered, or the evaluation of leftover stress whilst you're sleeping. The subconscious mind does some pretty amazing things at night. If that's the case, you'd not be in the best of places in the morning, for obvious reasons. Your mind may be a little off for a few hours, you might be a little tense, a little disoriented, a little adrenaline filled... Any of that stuff. It's a possibility. The idea with the serotonin is to get it to the ideal level. Depression is usually low serotonin, and anxiety and OCD high, although, a brain that isn't using serotonin effectively may be both. With anxiety, there's usually too much. The drug limits the amount that can be absorbed, making sure the level remains quite constant. This means constant, level mood. In this state, you're able to deal with things better, anxiety isn't usually the default state. Out of interest, why did your last doctor add another med onto the Celexa? What were your symptoms?