Laptop turns on spontaneously

I have a Dell Inspiron 1525 laptop, about three years of age. Usually I suspend it (put into sleep) in the evening and resume in the morning. Recently the laptop started turning on by itself during the night — not to mention that this always happens at about the same time!

The internet (e.g., this question) mainly suggests turning off Wake-On-LAN for the Ethernet card (I've long had it disabled through ethtool, let alone that there's no cable plugged in), or inspecting scheduled tasks. But I use GNU/Linux (kernel version 3.1.7, x86-64 arch), so the second option also doesn't seem plausible. The BIOS is at the latest version. I've checked the package manager logs and there were no suspicious/interesting changes during the period when this started happening.

Having spent some time looking into the problem online, I dare not ask for a ready solution, but for at least a suggestion of what could be wrong, and if this is likely to be a software or a harware problem. I imagine that it could be due to dust or partially broken/bent wires causing a short circuit and a pulse somewhere on the power button or other reset lines; but a software problem seems possible as well.

UPDATE: Thanks for the suggestions. Some additional info:

• dmesg has no suspicious lines
• The BIOS does not have (or show) any resume-on-alarm settings

My first thought is to check the BIOS settings for any kind of auto on setting. Most have a way of turning themselves on at a certain time of day, this may have been enabled. You could also go into BIOS and set defaults and reboot to see if the alleviates the problem as well.

As for less likely possibilities, like you said it could be a short in any of the circuits for power or reset switches, but also something in the keyboard as well since tapping a key on the keyboard will also wake up a sleeping computer.

The last thing that comes to mind is some really funky power issues. Spikes or sags in power can do some really weird stuff to computer equipment.

As @Paperlantern says, many BIOSes have a "power-on-at-a-certain-time" feature. I'm certain it is this, especially if it reliably happens at the same time each day.

If your BIOS does not have such an option, it's unlikely but possible the BIOS and embedded controller in your system support this feature, but it was just made inaccessible from the Dell BIOS menu by Dell. If the battery in your laptop is going out, or if a program messed with the CMOS NVRAM, the right bits may have been flipped to enable this. You might try updating your BIOS. If the battery is easily accessible (not sure about that specific model) you might try removing all power, removing the CR2032 battery, waiting about 15 minutes, and then replacing/reassembling and then go into your BIOS and configure it as you need.