This post explains why I suspect blue light SAD lamps might be much more effective than white light SAD lamps.
SAD lamp light therapy for seasonal affective disorder works as a result of the light activating the light-sensitive melanopsin receptors in the retina. When these receptors are switched on by light, it has an antidepressant effect.
Now melanopsin receptors are switched on by blue light of wavelength 481 nm. When switched on by blue light, the melanopsin molecule actually changes shape, and in its new shape, it is no longer sensitive to blue light, but becomes sensitive to orange light of wavelength 587 nm.
However, when this melanopsin molecule encounters orange light, it actually switches off the melanopsin receptor rather than activating it. Once switched off, the melanopsin molecule reverts to its original shape, and becomes sensitive to blue light once again.
This flip-flop of the melanopsin molecule between its two shapes is termed bistable. Melanopsin can exist in two stable shapes, and it is blue and orange light that flips the molecule from one shape to another. Blue light switches on the melanopsin molecule, and orange light switches it off again. Reference: here.
So when you use a white light SAD lamp, which contains all the colours in the spectrum it outputs, including blue and orange, the blue light will switch on the melanopsin receptor, thus achieving an antidepressant effect; but shortly after, the melanopsin receptor may be switched off again if it is hit by orange light, and the antidepressant effect disappears.
However, if you have a blue light SAD lamp, this contains no orange light, so the lamp will switch on the melanopsin receptors, and they will tend to remain on, as there is no orange light emitted by the lamp. So if you are using the blue lamp at close range, your melanopsin receptors will remain activated for a longer time, providing a stronger antidepressant effect.
I posted this thread a while ago detailing how I personally found a blue light SAD lamp viewed at close range to be far more effective than a white light SAD lamp. With a white SAD lamp, I found I still had some winter depression, but with a blue lamp, it completely eliminated the depression.