Take a 1mg melatonin around 10pm to get a decent night's sleep. Then the next day, stop drinking caffeine after 3pm and make sure you stay under 400mg. The reason why is because caffeine has a biological half life of roughly 6-8 hours. This means that if you consume 300mg at 3pm, you're still going to have roughly 150mg of caffeine in your system at 10pm. It's still possible to sleep after drinking caffeine, but the sleep quality gets worse, so you're still tired when you wake up.