You’ve been told since you were a small child to brush your teeth, especially as a child after having eaten a load of sugary junk during the day. However, that’s not technically the best advice.
While brushing your teeth regularly is advised, what isn’t is brushing teeth immediately after meals and drinks, especially if they were acidic. Citrus fruits, sports drinks, tomatoes, fizzy drinks all soften tooth enamel and brushing your teeth after consuming them only speeds up the acid’s effect on your enamel. This erodes the layer underneath, which you obviously want to avoid.
Colgate suggests waiting for at least 30 minutes after consuming anything to brush your teeth, meaning first thing in the morning is probably better for you than straight after breakfast.