Light SensorThe all new larger screen iPod touches don’t include an ambient light sensor for automatic adjusting of the screen brightness. In an email sent to the read of iDownloadBlog, Phil Schiller (Apple VP of Worldwide Marketing) explains that a light sensor cannot be accommodated in the device because it is extremely thin. There appears to be just no space for it.
Phil Mail
Here is what Phil said –

Thank you for purchasing a new iPod touch. It is a remarkable device! The 5th generation iPod touch does not have a built-in automatic light sensor (it’s just too thin!)

Image credit – iDownloadBlog