Unusual harmonic data reading from phototransistor in a simple arduino setup
The reason is probably an aliasing effect of your sampling frequency with the line frequency that powers some fluorescent or LED lights.

E.g. if the sampling frequency is alsmost a multiple of 50 or 60Hz (depending on your electrical power system), but not exactly: if the difference between sampling frequency and multiple of power line frequency is just 1/8.5s = 0.118 Hz you get this sinusoidal response.

See following image taken from Sampling sinusoidal functions:


The red sinusoidal line represents the variation of light intensisty by line frequency (50/60Hz).
The blue sinusoidal line represents your output.
The black dots represent your sampling events (it works also if they are even more spread as shown in the image; i.e. e.g. sampling in every n-th period).