Taking melatonin wrecks my deep sleep

I've been experimenting with melatonin. I bought Natrol 1mg and started taking half a pill about 1h before bed for some time, then 1mg, then 0.25mg. I have also tried it 2h before but almost ever it affects my deep sleep negatively.

There is of course a high probability of misinterpretation due to lack of data and other influencing factor. Nonetheless, any suggestions what else I could vary?