Question

give reason why one must not eat sweets after brushing teeth at night​

Answer 1

Answer:

If you have food (or a drink with a bacterial food source i.e. sugar) after brushing at night, this will alter how bacterial biofilms reestablish themselves while you sleep - likely in an unhealthy way, with more acid producing bacteria.

Answer 2

Answer:

After you brush your teeth; it removes all the plaque and food stuck on your teeth. If you eat sweets right after brushing your just getting food stuck on your clean teeth and also sweets taste bad after brushing your teeth, so eat first and then brush.