im 18 and have horrible insomnia, why is this?

[bob k]

for the past month ive been waking up around 9 in the morning and trying to go to bed somewhere around 12:30, trying to set a pattern for my sleep. for a few days ill be able to sleep through the night just fine and then soon after that there will be a night where i cant go to sleep at all. im up all night and dont feel tired till about 7 in the morning. These sleepless nights will last for a few days after that, few good nights of sleep will follow it. I have no idea why this is. I also exercise a lot, around 4-5 hours a day. There was a period of about a coulple of months where i was smoking weed 3-4 times a day but this was almost a year ago. Would this have anything to do with my insomnia? if not what could be causing it?