Chinchilla’s natural behaviour patterns are for them to search for food at night time in the DARK.  This is the safest natural time period to be out in the open and looking for food and safest from predators, and hence why they are nocturnal (or actually semi-nocturnal).  The hours of darkness are their feeding time.

The daylight hours are when Chinchillas are naturally designed to sleep and to rest.  It is the time that their bodies expect to recharge.  In covering a cage we remove that natural day and night time pattern of light and darkness.

A day/night cycle is vitally important to the long term health and happiness of any chinchillas that you keep as it play a large part in the control of their natural  body rhythms, feeding and breeding cycles.   A hormone called Melatonin is produced by the pineal gland and the intestine and has an effect upon their daily and seasonal cycles and rhythms. This hormone is only produced during the hours of darkness. Changes in the production of this, which can occur with insufficient light/dark patterns can have a knock on effect into their seasonal and breeding rhythms.  If chins have too much dark then too much may be secreted - too little darkness and their may be insufficient. Research as to what the effects of these would be has not been carried out but it has been noted that the number of litters produced when a day/night cycle is not maintained is dramatically reduced.
                 

Where chins are kept in the home, then the ideal situation is to maintain as near as possible a normal daylight and night time period.   If the room is dark, then leaving a light on during the day (preferably a northlight tube as this has the same spectrum as natural daylight) will help to keep that natural balance.  Most big breeders try to keep daylight hours of approximately 7am until 12 midnight - dependant upon their own circumstances.  (If using timed lights try and use a split system so that they come on staggered rather than all at once as this can scare the chins)

 If you are concerned about your chins privacy and a need to hide for comfort then provide a nest box in which they can hide but are still aware of the daylight outside their hideaway.  This stimulates the natural burrows or rock crevices they would use in the wild.

If chins don't get enough daylight hours or the balance of dark to light remains wrong, then this will affect their eating patterns, and also their breeding patterns!