Extreme anxiety.
For the longest time, I couldn't recruit enough concentration to get homework or big projects done until it was this huge looming threat. Frequently, that would involve an all-nighter since it was something due the next day. Other times, it meant cranking out last night's math assignment in home room mere minutes before it was due. It turns out that adrenaline and other stress hormones are great at shoving all the ADHD noise out of the way, however temporarily.