I tried something like that but after some help in irc chat I settled on this one.
cd ../home
sudo mkdir jb
cd jb
sudo mkdir scripts
cd scripts
sudo nano /home/jb/scripts/update
#!/bin/bash
apt-get update ; apt-get -y upgrade ; apt-get -y dist-upgrade ; apt-get clean ; apt-get -y autoremove ; reboot
sudo crontab -e
0 3 * * * sudo bash /home/jb/scripts/update