Силбней

The whole sentence is:

Ночью ты кричишь сильней

Which is translated (by my verb book) as "At night you scream louder".

What part of speech is сильней? It looks like a comparative but would that not be сильнее ?

I've come across another one of these recently also with the ей ending though I can't locate that one now.

спасибо

-ей is a common alternative to -ее as a comparative ending.

https://en.wiktionary.org/wiki/%D1%8...BD%D1%8B%D0%B9

тёмный

comparative (по)темне́е or (по)темне́й
superlative темне́йший

Сильней is a colloquial variant. So, you can meet other comparative adjectives with -ей.
More discussion is here https://forum.wordreference.com/thre...1489269/?hl=ru