Ubuntu: Dropbox and crontab

In your default user, run "crontab -e", and add the following lines:

0 4 * * * DISPLAY=:0 /usr/bin/dropbox start >/dev/null 2>&1
0 5 * * * /usr/bin/dropbox stop >/dev/null 2>&1

This will start dropbox at 4am, and stop at 5am.