Why doesn't the screen command source my .profile? When I start a new screen, the aliases that I have set in my ~/.profile do not seem to be sourced.

Does anyone know where I can change this or which file is actually read when starting a new screen?
For an alias to work in ~/.profile the session needs to be a login shell. Bash, which is probably what you're using, is typically read from ~/.bash_profile.

There's a good explanation of the differences mentioned here:

https://superuser.com/questions/183870/difference-between-bashrc-and-bash-profile