SmartCap measures "ability to resist sleep", which is a scientific term equivalent to a measure of the risk of a microsleep. When we voluntarily rest, we do not experience microsleeps. Microsleeps only occur when we resist sleep, and it is in this pre-sleep phase of resistance when SmartCap measurements are relevant.
If you choose to rest and sleep while wearing SmartCap, numbers will still show on the display, but these will have no meaning. That's because our brain doesn't switch off - so signals are still processed - however our processing algorithm only produces meaningful results when the risk of a microsleep is present (i.e. when you're resisting sleep).
Put simply, SmartCap is not a sleep detector, so there's not much use wearing it while you try to sleep.