2 issues here. 1st your computer's sleep mode settings & 2nd LW will go into "Idle mode" after some time if there's been no mouse activity. This has no effect on LW uplding or downlding. It simply closes off one of the connections which isn't being utilised. After all, if you're not at your computer searching then this aspect is not required.

For mac OS sleep settings go to System Preferences> Energy Saver & make appropriate changes to sleep settings for monitor & HDD. I have mine set to never sleep b/c with video work I can't allow the comp to go to sleep. If mac OS goes to sleep it's not always in the best of shape when it wakes up depending on various things such as which apps are open. Hope this helps.