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I had exactly the same with my 21mo. He started one morning about 6am. The next night, he wouldn't go to sleep alone and started waking in the night too. Then the day nap followed the same pattern. I don't know if they get nightmares - seems so young. My sis says they see ghosts .
I've got him back into a routine (out of cot, into bed, sit with him til asleep, slow withdrawal) by reassuring him.