How do I stop sleep changing to hibernate overnight?

Since I switched from Linux to Windows 7 a few weeks ago I have been finding that my Thinkpad R61i changes from S3 sleep to S4 hibernate overnight. That is to say, I put it to sleep when I go to bed, but in the morning it's hibernating and requires 10 minutes to wake up, which is unacceptable because I find the first few minutes after I get up a valuable time to write. The machine must be waking up in the night to do the hibernating because I don't have hybrid sleep enabled and it only takes a few seconds for the machine to enter sleep before my bedtime, not enough time to write to the hibernate file. How do I keep the machine in sleep overnight?

I should say that my third-party battery is showing signs of age and will occasionally falsely announce that it is empty when I am running on battery power, causing an emergency hibernation, but I always keep the machine connected to the mains when it is sleeping overnight. I don't know whether it is a connected or a coincidence that the problem has started since switching from Linux to Windows.