Could I connect a macbook pro to an external hard drive for use with time machine I want to know if connecting a macbook pro to an external hard drive for use with time machine when I am not using it, at night, will work. 

If I then disconnect it in the morning (regardless of the progress of the time machine OR journaling) will it cause any problems?

Thanks
There are third party solutions to schedule the timemachine backups. Backing up to any standard USB external harddrive should not be a problem at all.

While I cannot give you a firsthand review, TimeMachineEditor seems quite popular and reliable:
https://www.google.fr/search?q=timemachineeditor

When disconnecting the harddrive, you should however always make sure that the backup is finished and no write process is currently done. Eject the drive would be the safe thing to do.