When you get up in the morning you start using the energy you have stored up while you slept. It is common, I think, to "hit a wall" by the afternoon.
I have found that some days I hit the wall by mid-morning and need a nap and then I might need to nap again in the afternoon. I have had days when I only manage 2 hours at a time of good energy.
You might also find that your energy is depleted quicker due to trigger events which could be from something stressful to temperature changes. You will not get the energy back until you re-fill your energy levels. For me, I need a nap or some "me time" in my "fortress of solitude" (man cave).