Is this to regulate sleep only? And not depression?

Are you taking methylcobalamin? This form of B12 is the cofactor in the brain that is used to make melatonin from serotonin.

So having a good blood level of this, may help with sleep.
That is when I started methylB12 myself. My blood levels were in the upper end of the range- at 849, but during menopause I had real sleeping problems. The methylB12 at night helped with this.

Celexa is considered a weaker SSRI....many people who get side effects from the others, can tolerate it. Some even take 5mg which is a low dose.

I use 500mg of L-tryptophan at night with good results. I have some chronic pain issues, and it seems to help. I tried amitriptyline and it didn't work, and in addition raised my blood sugars. I use Doctor's Best brand. I had some by NOW that was not as active, as the Doctor's Best. I am taking 4 of my PN supplements from Doctor's Best now. This brand also beat out Swanson's for my L-theanine.

Doctor's Best is now available on Amazon, at a good discount and with the free shipping for $25.

It is not advisable to take any L-tryptophan with an SSRI...this may cause serotonin syndrome. So you have to be off the Celexa for at least 2 weeks or more.