Yes eating cabbage can make you sleepy.
Some people including myself do get sleepy after eating cabbage.
Cabbage has something in it that induces sleep in some people which can be a good thing if you have trouble sleeping and want to get to sleep naturally.
I can eat a little bit of cabbage before bed and I'm usually drifting off to sleep within an hour so of eating the cabbage.
Not everyone gets sleepy when eating cabbage but some people do.
Our bodies are all different and some people get sleepy from cabbage and some people do not.
For me I do get sleepy from eating cabbage.