We have one of those dimmer light switches that you tap to switch the light on and off and hold your finger on to dim or brighten the light. This controls the main overhead light in our bedroom.
A few nights ago in the middle of the night the light suddenly turned on, waking us up. I got up and turned it off without problems. The night before last it happened again!
I’m not 100% sure how these switches work. What could be causing this?
A moth?
Any chance it’s an X-10 compatible switch?
Maybe, although I haven’t seen any.
What is that?
I have one that does that too. I suspect it’s sensitive to voltage fluctuations. This site seems to confirm that theory.
Components in the electronics can be going bad.
Do your house walls bleed?
Have you stepped in something slimy?
In case of either the switch may be shorting out due to paranormal residue. Call 555-2368
This happened to me in a place I used to live. Same scenario: dimmer switch controlling the overhead light fixture, the light suddenly coming on without anyone touching a switch. It turned out that there was a surge protector strip plugged in to the same circuit that was going bad. Once I unplugged that, and replaced the strip with a new one, it didn’t happen again.
Disclaimers:
Your cats are planning on driving you crazy and making off with your inheritance.
We always blame the ghost of the previous owner (she died the day we closed on the house, and had Alzheimer’s. Our theory is that she doesn’t know she’s dead).
We’ve long suspect that voltage fluctuations have a hand in it, too. If you flick the wall switch that controls the outlet the lamps are plugged into, you can change the light level.
Seriously… do you have a dog or a cat that can get to the lamp? My dad used to have a Sheltie that figured the touch lamp out and she used to drive us nuts, turning the light off and on… off and on… off and on…
She seemed quite proud of herself.
I see dea…
Nevermind.
Shhhh… those are the NEXT steps! You’re spoiling the experience for them! 
Ghosts… or voltage variations… or you need a new switch.
I have a lit magnifying mirror with a touch switch and sometimes it will go on sponanteously if the cord on my phone charger is touching ANY part of the lamp (not just touch part, it will trigger if the two cords are touching).
They work by detecting capacitance changes – nothing happens if I hit the base of my lamp with the sole of my foot while wearing tennis shoes. I’m not sure what could cause those changes spontaneously though.
No pets in the house, although the switch would be too high anyway. Thanks for all the answers so far. I think the most likely cause is voltage spikes - I can’t see what else it could be.
The previous owner is alive and well and seemingly not distressed by the dead, so I am suspecting natural causes. 
Those normally rely on capacitance to detect touch. It is actually a tuned oscillator that is detuned by body capacitance.
The only time I’ve seen them cycle by themselves is when there is very high humidity - the light will continuously cycle up and down in brightness. But it is possible for transient noise on the power line to trigger it.
If this happens to be an old-style “Sensitron” unit (pre X10), it will have a cover plate with a large oval touch surface and a somewhat hidden switch on the bottom edge of the cover plate. If you pop the cover plate off, you’ll see a copper spring in the middle of the actual dimmer which contacts the cover. Check for dust / cobwebs / etc. between the spring and the surrounding metal area.
But be sure to unplug it first!!!
Good general advice. In the case of the particular Sensitron unit I’m talking about, the cover is a snap-on decorative cover which just contains the touchpad surface. The actual guts of the are behind another aluminum panel, with the only protruding part being the sense spring.
This is very interesting to read for me, because I have a similar situation:
a unit that controls a light and also the ceiling fan in the same fixture. The unit has 4 buttons: 3 ceiling fan controls and one light control.
There are three buttons for the ceiling fan: high, medium, low.
For the light switch, you can either press the light button to turn on/off, or else you can keep your finger on the light button to have it go from dim to bright and vice-versa.
Sometimes I will be sitting in the room and the ceiling fan will come on by itself. I have the same system in two rooms and it only seems to happen in one room, and only in the summer. This is the room that gets the warmest because of the way the windows face. Could it also be because of voltage fluctuations?
This is how my switch works exactly. When it turns itself on it goes straight on and stays there - the brightness isn’t adjusted in any way.
The reason has already been given - fluctuations in the power line - , I just wanted to add that this reason was told to me by an employee (IKEA) in the lamp shop who sells touch lamps, and has heard from customers in some areas (where apparently the power varies more) that the lights go on during the night, and how puzzled they were.