Is it harmful if i charge my iphone 5s all night ??

No. In fact, it has some advantages. If your phone is plugged in, connected to WiFi and the screen is locked, it will automatically back up to iCloud. And, it will be fully charged and ready to go in the morning.

Meg St._Clair wrote:

No. In fact, it has some advantages. If your phone is plugged in, connected to WiFi and the screen is locked, it will automatically back up to iCloud. ...

If Settings > iCloud > Backup is turned on.

(It's a rare occasion when I see the opportunity to enhance an answer from Meg, so I'm not going to pass up the opportunity!)

Thanks! 🙂

Always much appreciated.

Glad it helped!

And (simply to take the opportunity to amplify Megs answer) you cannot charge it all night because the charging circuitry shuts down once the phone is fully charged. Plug it in as often and for as long as you want to.