Great job on the heads buzzin-bea, you managed to capture a lot of character in such a short amount of time. Just keep in mind that there's only so much you can do in one minute. Producing good drawings requires time. You should consider mixing shorter and longer time intervals to get the best out of both. Keep it up!
Just to know:
1 minute is a long time. Have you made for the whole of the board all these portraits?
If so, that's good.
Otherwise also seek to develop a head only 1 minute. At that time, you will not have a sketch but a beginning of drawing.
Anyway, I like what you have presented.