Is it safe to take melatonin every night?

I wouldn't take any drug every night. Drugs tend to signal to your body to reduce the production of that particular enzyme, hormone, or neural transmitter. In the case of melatonin, I'd say that your pineal gland might start reducing production of it as it gets signals from your body that you already have sufficient amounts. Even if those "sufficient amounts" are actually insufficient—in which case you might need to find out the root cause as to why the gland is signaled for less production from another cause.
Of course this is not doctor's advice. A doctor is your only official source of advice.

I personally think drugs of any kind are only to be used in dire circumstances. Even caffeine. Drugs do have a place in health, and anyone that has ever driven in caffeine-restricted Utah can just imagine why the state has so many drowsy driving billboards all over their interstates. People are probably getting into accidents because they're afraid a cup of coffee at a particularly strategic moment will violate church teachings.