Yellow leaves falling after 5 weeks into flowering

[h33xxx]

Hi there, I have 3 plants that are 5 weeks into flowering and the leaves at the bottom are turning yellow and falling.

Is this normal? Is it a NPK deficiency?

The plants have been growing behind my living room curtain (window-grow), but i dont think it's lack of light.

:(

[Rusty Trichome]

A picture would help. Yellowing in late flower is normal, but it's a tad early for that.

The window is a problem. Not nearly enough hours of direct light, and how do you keep them in total darkness during the darkness period?

[h33xxx]

I will post pic ASAP. I dont think light is a problem because the plants get like 13 hours of light and at night they have total darkness