Hello everyone! The baby is turning 5 months old and started having…

Hello everyone! The baby is turning 5 months old and started having trouble sleeping both day and night. He used to sleep well before but now he's fussy and won't take him during naps or at night, he cries as if something isn't right. When awake, everything is fine, he plays, talks, laughs and doesn't cry. It's only when he wants to sleep that he starts being fussy. What could be the reason for this?