Lack of sleep or feeling tired the next day - I guess it could be from:
1) Skipped breakfast
2) Chat into the night
3) Watch a movie
4) Play games on your smart phone
5) or TED didn't let you sleep :-)
How many of us remember basic Math from High / Jr-High school involving Calculus, Probability, Geometric equations or data structures? Personally I cannot recollect much of it. Is it our carelessness to remember stuff or the learning practices implemented by our education system responsible for it?