Why is it that when you are anxious or excited about the next day you can't fall asleep?

because you're mind is restless. if's like trying to sleep when you're watching a good movie or having fun doing something. one must put his/her mind at ease OR be physically worn out to fall asleep.

Because this primes you for action and this is the reverse of getting calm and failing asleep. You need to think about nothing or something else. Set up a TV and watch something in bed, that should calm your mind, or read a book in bed.

Getting excited or anxious stirs up and disturbs your nervous system, that's why u can't fall asleep, try to take it easy.

When you are psychologically excited, it leads to physiological arousal. Your nervous system (starting with your brain) is excited, and so it increases your heart rate and breathing slightly, and might even increase your resting adrenaline levels. Because your body is sort of "wired" due to your brain activity, it is hard to get both body and mind to calm down and allow sleep.

i totally know what you mean!! Whenever you are laying in bed and the next day pops up in your head, think of something different. instead of thinking of the teacher, think of all the funny things that happened last school year. and when you think of the game system, think instead about going over how you will be putting it all together. whenever i lay in bed thinking. i just go over a dance routine in my head!