To clarify, the client computer usually goes into sleep mode at night, and it appears that after a lengthy "sleep" period (ie until the next morning or after the weekend) it loses the AC database connection. However, if I put the computer to sleep and quickly wake it up, it somehow retains the connection.
This must have something to do with the "hibernate" setting that is on by default in many laptops with Windows 7, but was not as common in Windows XP. It might be possible to prevent the laptop from hibernating in the Power options Control panel. Or as Jon said, don't shut down, just close the laptop to turn off the display.
