...I'm just wondering if when the Mac sleeps the transfer is still progressing.
No. Your Mac got bored and the transfer stopped. Set it to "never sleep".
This is from Mac OS X v10.6: How to use Migration Assistant to transfer files from another Mac but the principle still applies:
- Choose System Preferences from the Apple () menu.
- Choose Energy Saver from the View menu.
- Select Custom in Optimization.
- Adjust the slider to Never for "Put the computer asleep when it is inactive for..."
Don't use wireless for the transfer. Connect the two computers with an Ethernet cable, either to each other directly or through your router. Wireless will take much longer.