Own Laser helmet: lasers shine a lot less intense after +/- 15 min. Overheating??

[crypto]

2years after a 2100 grafts HT I have built my own very beta version of a laser helmet.
I have about 200 (5V, 650nm, 5mw) diodes and a 5V and 4A(!) adapter.

I've been using the helmet at least 3 times a week for 3 months now but it's hard to see any results to be honest.

What strikes me is that for some weird reason the lasers start with very high intensity and end with a lot less intensity after the necessary 20min. What would be the cause of that?

Thanks.

Enclosed: pic of 'dimmed' lasers...