This happens to me too. I think it may be related to dehydration. It usually happens when I've had too much coffee and not enough water...

My understanding is that the tiredness comes from adrenal burnout. Love to read other responses.

I experience something similar and have found that I can feel when that next cup of coffee will not wake me up, if not make me more tired. I've found that what works beter is getting up and moving around as well as drinking some cold water. Why this happens I'm not sure, but I can say it doesn't happen when the coffee is spaced out by 5 or 6 hours.

The same thing happens to me. I've been paying close attention to this for a long time. I think it has something to do with an allergy or intolerance to something in the coffee other than the caffeine. I can drink a lot of caffeine from other sources without problems. But too much coffee makes me dehydrated (more than normal), gives me anxiety, and makes me lag.

I feel like coffee gives me dirty energy. Like coal. It's a shame I love it so much. I have been told that if you have a mold allergy, coffee acts in much the same way as mold. Food for thought anyway.

My guess would be that it's not a coincidence--it's probably tied to something dietary or lifestyle (sleep patterns, wake-up time, exercise routines)--but that it's probably not tied to the coffee. Being that it's time-based, there could be any number of triggers that cause sleepy feeling shortly after 10:00. Since coffee does not have that effect on you at 5:30, I wouldn't think it would at 10 either.

Look at all of your dietary and lifestyle habits from the previous 12 hours or so, not just the coffee.

And it's always a good idea to go for a walk if possible. Movement helps get you awake.

When you consume caffeine it keeps your natural "downers" (adenosine) from limiting the rate of neural activity in the brain, hence the stimulating effect. However, with repeated exposure, the effect gets smaller as your body learns to anticipate this stimulant load and acts to mitigate this effect. As far as I know, research on the mechanism for caffeine tolerance is ongoing but is focused on sensitivity effects in adenosine receptors.

You mentioned your second cup was half the normal size. It's only my speculation, but it's possible that your body was anticipating (and compensated for) the caffeine load in a full cup of coffee. This lines up with what's known as "the situational specificity of tolerance" which has been observed in response to caffeine:

http://circ.ahajournals.org/content/108/6/e38.full

Feeling tired after taking a stimulant could also mean that your body is simply overstimulated in general, and that the stimulation side of the caffeine-balancing system is maxed out.

Is this a one-time thing or a recurring phenomenon?

One-time thing: Hard to say it's more than a coincidence.

Recurring phenomenon: Hard to say what it is, could be a myriad of different effects. It's N=1, so it's up to you what you'd want to do about it. After all, many people (myself included) drink 2-3 times as much coffee in a day as you do without ill effect.

Several factors can cause this from coffee, coffee stresses the adrenal glands and gives you a false energy boost, over time those adrenals will become over stressed so you crash. Secondly coffee dehydrates your body. For every cup (not mug) of coffee you should drink 2 glasses of water. Recommended safe dose of coffee is 3 cups per day (not mugs or paper cups). It is not how much coffee you drink, it is for how long you have been drinking it. Coffee also upsets the stomach and intestines and makes them irritated. I recommend switching to green tea for a month. Green tea will hydrate you. But only drink 2 mugs per day, as green tea as been shown to stress the liver and kidneys. Drink about 2 liters of water per day for a month. Eat healthy, lots of fruits and veggies. Listen to your body.

I used to drink 5 to 6 mugs of coffee each day for a long time and do cardio workouts. Not a good idea. One day after my 5th coffee mug and exercise my body just died on me and i was about to faint. Luckily i was able to get a hold on some water and hydrate myself. But i do not wish this experience for you.