I'm thinking of giving my 5-month-old a bottle of formula at night before he sleeps so he'll sleep longer. Been exclusively breastfeeding all this time. Is there really a benefit to exclusive breastfeeding vs. "mostly" breastfeeding?

In fact, your baby's night waking doesn't really associate with whether his tummy full or not. Some babies wake up at night to seek for comfort. If you suspect the night waking is due to hunger, you can tank up your baby during the day by nursing more frequent instead of introduce formula milk. There is nothing wrong to feed formula milk. However, when you read up the label for the ingredients from formula milk, you will realise actually the major ingredient is the corn syrup (sugar) and many other artificial supplements. Since you have the best milk for baby, please do continue to offer breastmilk.