Sleep regressions are caused by developmental milestones which can interrupt the normal sleep patterns. Learning to stand, crawl, sitting and walking is a physical milestone all babies go through.

Spend some time in the day when they are not due for a nap, learning to sit back down and lie down.

Wait and listen before responding at nap time and overnight and if not crying and they have the ability to self settle then just leave them to it and let them yield to that sense of tiredness and going off to sleep.