AnalogRead returns Waveform

While toying with some of the analogReads, I noticed that my A1 that was connected to my breadboard (a point that had no attachment to any circuit) would return a waveform pattern (see attached image) - I got that by charting the output against the time lapsed ( using millis() ), and estimated the difference between each waveform to be ~3 seconds each. Could someone enlighten me on what exactly I'm measuring with this? Initially I thought it was the AC; but then I thought that runs at 50Hz so this waveform is slower than that.

It might be any device with imperfect shielding somewhere around your setup. A floating pin might take up any ESR from any electrical device that emmits some fields.

I don't think that's it - I'm running it off my laptop, and moved to a room that hasn't got any other electrical devices, and placed the max distance I can from my laptop and the board. I've also tested on 3 other analog pins, which all return identical waveforms. I'm assuming its something to do with the arduino electrical supply, but my electronics knowledge isn't good enough to answer the question :confused:

Post a photo of your setup and the sketch you use for testing. It might be the waveform of the USB power but that depends on the code you use.