CBD isolate was shown in studies to improve REM sleep, but it does not make you feel tired or help you fall asleep. The feeling of sleepiness with CBD is listed as a negative and rare side effect. Full spectrum can contain other cannabinoids and terpenes that can make you feel tired. So the better question is, are you taking it to feel tired or to get better quality sleep? And an even better question: do you need to pass a drug test?