When we wake up in the morning we seem to shift from a world where we have full control (or our subconconscious has full control) to a world where we likely have very little control over what happens. This can feel like a shift from contentment in to a depression of sorts, and so often in the morning we are not so happy.
It appears if we practice accepting this shift as inevitable, and embrace it enthusiastically - we have to get up and face the music some time, then it seems we can avoid an overly negative start to the day. The fuzzy cloud of disappointment - discovering we are no longer relaxing - lifts soon enough once we get our metabolism, etc. working, so why draw out that disappointment and hide under the duvet?
Rising with the Sun - joining the day on it's transition from morning to night - can also seem to give us a sensation of being integrated with the planet on which we live - a positive feeling of being a part of profound cycle.
Many ancient wise people spoke of waking up from a world of self-delusion; as if we walk around sleeping for most of our lives even when not in bed. The more we gain an appetite for waking up in general, maybe the more we will begin to face and enbrace the reality of our condition.