I don't really have answers for you but I've been experiencing this myself. I've been working out 6 days a week and eating 1500-1800 calories per day. I have energy during the day but once I get home in the evening it's a struggle for me to stay up past 9 pm! I used to go to bed around midnight! But I have to say it feels good to get all this sleep
What I think is happening is that my body is just not used to having so few calories and it's hard at work burning them off! Calories=Energy so if they are being used up it would make sense that we would have less energy.