Most people swear by the sedating effects of smoking pot. I personally think it doesn't actually make you sleepy but puts you in a frame of mind where it's just easier to go to sleep. According to Elite Daily , recent studies have led to a more in-depth understanding of the effect of weed on sleep.

Source - howtoroll

There are two kinds of sleep - REM ( rapid eye movement) sleep and non REM sleep. REM sleep occurs around 90 minutes into a nap, and is a period of light rest during which we dream and our cognitive selves ease up, so to speak. Non REM sleep is the deeper slumber during which our bodies heal themselves. There's apparently quite a bit of evidence proving that marijuana suppresses REM sleep, which basically means that you barely have any dreams, or recollection of them.

Source - huffpost

Not dreaming, while being a great loss in terms of creative and amusing sleep, can also be a blessing for people who are plagued by nightmares, perhaps due to PTSD or some other underlying issues. If you suddenly stop smoking however, your dreams tend to become that much more vivid. Several war veterans have sworn by the nightmare eliminating effects of weed, especially in comparison to other chemical anti-depressants.Even America's top doctor, US Surgeon General Dr. Vivek Murthy recently stated, "For certain medical conditions and symptoms… marijuana can be helpful."