<https://www.mumsnet.com/Talk/sleep/4118065-Older-sibling-sleep-regression>
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"Hi, my DS is almost 3 and has been a good sleeper. Goes to sleep himself and is out for the night. However, at the end of October his baby sister arrived. She's now 8 weeks old and since last Sunday DS is crying out for daddy as soon as we leave the room at bedtime. We've tried reassuring him through the door and leaving him to it and going back to check at intervals but he just works himself up into a frenzy. The only thing that works is to go into him and cuddle him till he sleeps. He's also waking up in the night and doing the same thing. He says his sister wakes him up but we try and get to her before she makes too much noise but I think he hears us get up to make her bottles. Has anyone else experienced anything similar? If so how long did it go on for and what did you do to fix it? Im not sure gong into him is the best thing as he'll expect it everytime but on the other hand we all need sleep and I hate to let him scream as he can keep it up for hours. I'm assuming it's all to..."
<https://www.mumsnet.com/Talk/sleep/4118065-Older-sibling-sleep-regression>