You really should go to the doctor. You could be anemic. Especially if you take antacids for your reflux- they can prevent absorption of B12, which can cause extreme fatigue. Or you might have cancer. Or something. Go to the doctor.
I agree with those who suggest a sleep study. Apnea is not the only issue it might diagnose. My wife doesn’t typically reach the REM stage during sleep - they found that out during her sleep study.
This used to happen to me but it was because my body got used to being at rest at 6 pm when I got home.
In the end, I just had to push through it every night for about three weeks. Then my body readjusted and I felt normal when I got home from work again.
Additional question, how long is your commute? I find there is an optimal commute time for me. If it takes under 10 minutes, I am still mentally ‘at work’ when I get home (which is no good for the kiddos). If it’s longer than 30 minutes, I find myself very sleepy (since driving is both boring and stressful at the same time).
I’m 4 miles from my office, so it’s about a ten minute drive. Although occasionally I run home from work, and then it takes a little longer. 
Okay as soon as I said that, my mind started racing. Something to the tune of “How come you always have enough energy to run 4 miles after work when the occasion calls for it?” And I’ve come to the conclusion that maybe working out after work really does make me feel more awake, and I need to consider that suggestion more seriously.
Another thought:
Back in February, I took five days off of working out, and it made a big difference in my energy levels (the last time I took that long off was in 2007). Perhaps I’m just not supplementing my diet properly to keep up with my workout regimen?
clears throat
Well…more carbs perhaps? 
… or iron, I wonder. If you’re a moderate meat eater and stick to mostly white meats, maybe the dark leafy greens need a boost, or a supplement.
If you don’t feel you could be snoring (do you wake w/ a dry or sore throat?), you might still be having your sleep broken up as someone suggested upthread. You’d get enough rest to get you through about lunchtime but that’s it. Are you aware of waking regularly at night? Do you get up to go to the bathroom at all?
Have you tried Melatonin? On the rare occasion that I absolutely MUST get several hours of good rest, I take a Dramamine and I won’t wake up once.
If you have trouble falling asleep after more than 15 minutes, there are simple meditation techniques you can choose from to calm your mind enough to drift off. I must read before I lay my head down or I flop like a fish for a half hour or more.
I agree w/ the suggestion to avoid eating after 7 and add not drinking after you have your last bathroom stop for the night. Good luck, it must be very frustrating for you.
A Dr. visit and blood work is required to Dx your problem if you have one, that is. Some people just need more sleep. Are you thin? Could be hyperthyroid–and so you are burning up your carb intake and then your blood sugar falls. This was my wife’s situation.
It turns out I have a low red blood cell count, of which fatigue is a symptom. While this is often due to low iron levels, the same blood work that indicated the low red blood cell levels also indicated that my iron levels were at the upper end of the normal range, so I don’t think iron supplements are the solution.
Online sources suggest making sure to get a protein rich diet, and since I originally posted this thread I have been making more of a concerted effort to have a full dinner, rather just picking at food in the evening, and this has seemed to help.
I have also introduced caffeine into my diet again. I am limiting it to one cup a day, but on days when I have plans in the evening, a cup of coffee helps me stay awake until bedtime.
ETA: Props to Rachellelogram for calling it.
amg! What do I do with this prop!? 
Glad to hear you got the problem diagnosed. Did you end up changing your workout times, too? I was wondering if that might help.
Nah. I tried it out for about a week or so, but I didn’t like it as much so I went back to working out in the morning.
experiment with caffeine. find a dose (strong, medium.weak,decaf) and a time(s) to have it in the afternoon. just enough to get you past your fatigue point without disrupting your night time sleep.
Already did this. My sweet spot is between 3:30 and 4:30. I haven’t experimented with the dosage, though, that’s a good idea.