The answer to the question is insulin response. Being as insulin is the primary hormone involved in the function of fattening up for winter, it's also an energy regulation hormone. It can act for stored energy in the body like a freeze works on your bank account where you can put money in, but much won't come out. This is one of the primary things behind why obese people tend to be tired, and why diabetic comas happen. Insulin prepares you for hibernation, makes you sleepy. This is also the real reason there's the occurrence of people passing out after Thanksgiving dinners, it's not the trace amounts of tryptophan in the turkey, but the 700g of carbs we just ate.
So on the flipside, when insulin sensitivity is restored, all of the other hormones flow and function better. Particularly the adrenals. This is why even in studies based on starvation, when people's blood-glucose got low and they went into ketosis by default, they were more energetic with less food coming in. They weren't defying the laws of thermodynamics as some would suggest, the stored fat was being tapped for energy and the machine was becoming more efficient at fuel regulation, while the fuel supply lasts.
If you think about it, the lower blood-glucose goes (and insulin goes), as a triggering mechanism to invoke storage (hunger) and energy conservation (sleep), the less it's being secreted, the less you'll feel both. Meanwhile fat provides for better hormone production all-around, including the adrenals. So yes, to feel more energetic, want to sleep less, and feel less hungry is plausible. But it must be noted that it isn't a natural biorhythm to live this way all year. Less sleep is over all a very unhealthy thing because it doesn't allow the body time enough to get into the cycles that restore immune system health to full function.
