Nursing can be very tiring on a baby's cheek muscles, which may contribute to their tiredness. This is quite common, especially for newborns as they get accustomed to this new way of obtaining nutrients.
Breastfeeding releases the hormone cholecystokinin in babies, which results in a sleepy feeling. Breast milk also contains other sleep-inducing substances that can help babies develop their own circadian rhythm. Newborns are more likely to fall asleep while breastfeeding than older babies, as they require more sleep at this age.
A poor latch can also cause a baby to lose interest in nursing and fall asleep. If this is the case, try switching the baby from one breast to the other when you feel them nodding off.
Is it different for older babies?
Eventually, all babies will wean from breastfeeding, and as they do, they will start to fall asleep without being breastfed. This also comes with sleep training. By establishing a sleep routine, the baby will learn different techniques to get relaxed and fall asleep more easily. This differs from child to child, so every family can establish their own routine.
The sleep-inducing hormones produced by breast milk are higher during the evening, and this, naturally, makes it easier for babies to fall asleep. The act of breastfeeding also makes them feel warm, safe, protected, and loved. Breastfeeding can help babies develop their own circadian rhythm, which will be helpful during sleep training.
On the other hand, your baby may not learn to self-soothe if they are constantly being nursed before bed. You could face a similar challenge with teethers as well. The breast becomes a replacement for self-soothing, and your baby could start relying on being nursed. It can also make weaning more challenging.