It's strange that people use hot chocolate as a sleep aid, given that chocolate contains multiple stimulants: caffeine, theobromine (powerful enough to kill dogs and cats at low doses), and PEA (amphetamine's closest relative naturally in our brains). For that matter, in college the single most effective remedy I knew for making myself focus and work when absolutely necessary was a packet of dark chocolate M&Ms. (I have ADHD, but I didn't have a prescription for that and am proud that I never once "borrowed" pills from friends, when many did who needed to far less.) The little glazed chocolate lentils would keep me up all night studying. It worked better than any amount of coffee. So it's a little peculiar that people use this as a bedtime soporific...
Well, maybe it isn't, actually.
Too bad my dark chocolate M&Ms trick would only work once. Afterwards, that emergency fallback wasn't a fallback for a while. (This was, not too surprisingly, a gigantic problem. ADHD is called a disorder for a reason.) If I tried in the next few days, I'd be crunching new bags all night, and I'd never get as focused, and wouldn't get half as much done with 6x as much M&M in me. To be candid, I probably wouldn't get a quarter as much done, even if I was panicking about my grade and fascinated by the subject.
Tolerance happens quickly. Brains are all about adapting. So is life. But you don't see people downing espresso as a nightcap (not often, anyway), let alone a pot of drip coffee. So... why hot chocolate?
See, there's something called a paradoxical effect. I'm sure you've experienced it. Maybe you drink a cup of coffee expecting it to sharpen you up, but instead, you start falling asleep, regretting the coffee. Did it just not work? Are you imagining that it made things much worse? This is most likely to happen when you have less than your usual amount - meaning whatever your system has adapted to over the last few days. I find adaptation can happen shockingly quickly, as in the chocolate example. If I drink a decaf coffee, that normally wakes me up. (Seem odd? First, I'm naturally very sensitive to caffeine; second, I rarely use it because it doesn't wait until my birthday to gift me lovely migraines; and third, decaf has caffeine; so yes, under those conditions it works exactly like normal coffee.) But if today I drink a normal coffee and tomorrow I retreat to decaf, it'll probably make me sleepier than no coffee. That's called the paradoxical effect. It happens with a variety of stimulants, and other medications as well: whatever your system is expecting the chemical to do, your system partly counteracts that; but if the dose is relatively low, your system's counteraction will be stronger than the effect of the chemical. Voila: coffee made you sleepy. Paradoxically. But very logically.
So - when you aren't used to caffeine, or chocolate, chocolate can be very powerful, actually. A large bar of dark chocolate has, depending on the size of course, more caffeine than any coffee drink. And don't forget its other active components. But milk chocolate - and especially chocolate milk, which usually doesn't have much chocolate - can easily fall into the range that's less than what you're used to. If you're used to sodas and coffees, a bit of chocolate milk might actually make you sleepy.