Low energy days happen to everyone. But consider what happens when you're low energy. For one thing, you are less animated. Your voice doesn't carry as well. You miss out on little things like emotion and tone in your voice. It ultimately boils down to being less engaging. Less engaging as a GM means that it is harder for your PCs to get into the game. Even if they are high energy, with nothing to latch onto that energy can't focus and you can end up with even more distractions.
High energy can be the opposite, but you can also have too much. If you can't just sit still you may have problems focusing yourself. The idea is to be driven and good to go, but also focused. Not restless but ready.
The next question becomes how to maintain your energy level. How do you make sure you have energy for your game? There are a couple things you can try, some - again - are very basic.
Rest & a Schedule
Having your game be on a regular schedule can help a lot. Humans are nothing if not creatures of habit, and knowing that every Friday at 6:30 you GM can help your body be ready to kick things into gear come that time. Proper rest is another obvious one. If you get a good night of sleep going into the day you run game you're going to be better off than if you don't.
Avoid Heavy Meals
A lot of people have their game over dinner. And for some people bring their own food, while others have a group meal. Either way, you want to avoid having a heavy meal. A full belly of hot food that's loaded with carbs is great...for going to sleep. Once you start feeling lethargic getting back in gear for game is going to be hard.
I am not saying don't eat dinner. Just try to keep it from being so heavy you put yourself into a food coma.
Energy level is as much about actual energy as it is about mentality. Rituals are a great way to control your mentality. Have a process of things you do that starts 5 minutes before game or however long you need. Get your notes together, review the game, review your prep for this session, go over what you have to do. Have a way to signal to your players that you're starting, and do it.
I've found closing a book loud enough to get attention can work wonders. Matt Mercer seems to clap his hands to get everyone's focus for the beginning of Critical Role. I have a friend who will just ask someone to start a recap of the previous session. Whatever works. It lets people know its go time, and doing it the same way will help everyone get in the right mindset.
Take Time If You Need It
Some days life is going to suck and you're not going to feel it. On those days remember two things: it's ok to not be high energy for every session, and it's always ok to take some time before game starts to take care of yourself and do whatever it is you need to do.
If you're an extrovert and recharge through social communication, hang out with your players for a bit and shoot the shit or whatever. If you're an introvert and need isolation, then get that for yourself for a few minutes and refocus. Your players will understand if you need ten or fifteen minutes to get sorted for game. Worst case scenario, there's always the bathroom.