R4sty:
But, what kind of arduino items can switch automatically the light?
Typically you can drive a lamp with a Relay (it's power capability need to match the system you control and of course you need to protect it).
Don't play with 110V or 220V if you don't know what you are doing, that can kill someone, burn down your building etc....
if you want it automated, you might want also to buy a Real Time Clock (RTC) to know what time of the day it is. Go for a DS3231 for example, it won't loose too much track of time.
if you have Wi-Fi where the terrarium is, you could consider using an ESP8266 or ESP32 or Arduino MKR Wi-Fi and get the time of day from the internet (but going to the internet for this is not energy friendly overall)