Babies will typically lose between 5% and 10% of their weight after birth and so need to gain this weight back. This is quite normal and most babies have gained the weight back by the time they are two weeks old.
Therefore if your baby sleeps more than four hours it is recommended to wake baby to feed them including during the night for at least the first two weeks, or until they are over their birth weight.
It is normal for your baby to wake at night at least for the first six months, sometimes longer, and this may be difficult to cope with when you are tired.
Speak to your midwife/health visitor if you have any concerns about bed sharing and safe sleeping.