Which user characteristic may NOT be used to change keyword bids in AdWords?:
time of day
It comes under the bid adjustment. Bid adjustments allow you to show your ads more or less frequently based on where, when, and how people search. For example, sometimes a click is worth more to you if it comes from a smartphone, at a certain time of day, or from a specific location.
Read more here: https://support.google.com/adwords/answer/2732132