I took my four-month-old son in for his four month check up today and found out some useful information I thought I would pass along. Reading many posts on here I was afraid to sleep train fear of regression. I brought it up to my doctor thinking it was something physical my baby would go through. Wrong! He said sleep regression is more environmental. Like, if you were traveling, moved, new baby, different loud environments. That is what causes sleep regression! Such a relief, back to sleep training!!