Can anyone shed light on what happened with my 5 month old last night?

Hi there,

FTM with a 5 month old, recently sleep-trained with CIO. He took to it really well and only cried 5 minutes the first night, then 30 seconds the next. After that, he was putting himself to sleep no problem.

Wake windows usually go 2/2/2.5/2.5, and his 3 naps are not great, usually ranging 30 minutes to 1 hour.

Last night when we put him to bed, he cried for 50 minutes (we checked on him multiple times). He was clearly falling asleep briefly, but would wake up with a start. After 50 minutes I went in and held him, and he was subsequently WIDE awake from 8pm til midnight. Babbling, cooing, not unhappy seeming, but wide awake.

Has anyone encountered this before? I am worried I’m missing something and would really appreciate insight from the worldly wise.