Why is my oral dose of meth only lasting 4 hours?

I have been wanting to try low doses of meth compared to adderall that always gets me jittery and mad and a hoooorrrrrible headache. I think I am finding my sweet spot and I’m very much into making sure I’m getting water, electrolytes, eating…especially bananas and protein shakes and then I have Gatorade and an electrolyte drink. And I’ve been reading Reddit and I think an mg scale will help out a ton! Soooo I say all that to say that when I am on here, I am seeing people last 8 to 10 hours… Is it because they are probably taking more than me? I took mine at eight yesterday morning and I started to come down and get tired and yawn around 12:30-1pm….i ate and got hydrated and took another low dose. Again, I am not complaining about the experience during…bc it’s aaaaamaaazing!! I’m just confused as to why it is literally only lasting four hours every dose…should I up it once I get my scale? Thank you all