i had just bought my 6months old sugar glider, he always sleep during day and night, inactive and doesnt eat the food that much. he only want to eat food which contain sweet taste. is this cause from depression or sickeness.

Make sure the room he's in gets light during the day and dark at night. Some gliders won't come out to play unless it's really dark.
He could be lonely, or could be sick - a vet check is never a bad idea. But you should get him a friend:)

I am assuming you got him as a solo glider, is that correct? Was he part of another colony before you got him or has he always been alone?
Depression could absolutely be a cause for the lack of appetite and loss of interest in playing at night. My first glider started showing signs of depression for about a week right before we brought home his cage mate. He stopped playing at night, he barely ate, he seemed almost lethargic, which scared me half to death, and left me hoping that he would be okay until we could bring the new baby home. Once the new baby came home it didn't take long for him to realize there was another glider around and they started barking back and forth all night from opposite ends of the house.

The first thing you want to check is output. Fecal and urine. If your little boy is dehydrated then he won't be using the bathroom nearly as much. You can also do a tent test to check that. Calling a vet either way is just a good idea. It is always better to be safe than sorry with these little guys. They hide illnesses really well from their humans. And by the time you see signs, it's usually too late. :(

i got a pair. male and female