I have taken 2.5mg Letrozole and the sudden drop in estrogen levels in my body causes insomnia. I haven't been able to sleep in one week. I should have gradually increased the dosage starting from 0.25mg to 2.5mg in one week instead of taking 2.5mg immediately.

If you plan on taking letrozole start with a low dose of 0.25mg and gradually up your dosage to 2.5mg to prevent acute reduction in circulating oestrogen levels which may lead to insomnia.