Three More Thoughts
There are probably a few more reasons. First, the blood vessels in your nose during a blow session retreat so the last lines are absorbed in the smallest amounts. Also, after an oversaturation of dopamine from the beginning of the session, the dopamine (D2) receptors retract. The brain tries to maintain stability so it will so the final lines may keep the same amount of dopamine in the synapse but the post-synaptic receptors are in lower numbers after an evening with yayo. Finally, the crash from the previous lines builds up and so the lines at the end of the night if they do much at all will just make someone go from exhaustion to feeling normal. I hope this helps.