Your body uses hormones, chemical messengers, to tell the body when to do various daily functions, one these functions controlled by hormones is our sleep wake cycle. When we sleep, all of our “stay awake hormones” like cortisol, adrenaline and histamine get “shut off” to allow our sleepy hormones like melatonin to keep us asleep so we can rest. When we first wake up, our body still has this imbalance of sleep vs awake hormones because just like some caffeine you drink, the effects of hormones can’t just stop on a dot, they have to gradually fall off.
When it’s time for the body to wake up, your wake up hormones start gradually rising, and your sleepy hormones slowly start lowering. When you actually wake up, the wake up hormones have just reached a high enough threshold to get you awake, the minimum hormone needed, but the sleepy hormone still isn’t fully depleted yet, and so when you wake up first thing in the morning, although you have enough wake up hormone to open your eyes, the sleepy hormone is still in your body, just in a small concentration, but it’s not enough to knock you out, so instead you just feel sleepy and “groggy”, and you have to get up and start the day and eventually the sleepy hormones will ware off. An easy way to think about it is same way you feel after waking up from surgery, because the sedatives in the Anesthesia are still in your body, they’re just in lower concenctrations, so they can’t knock you out but they still try to give it there best shot, leaving you feeling sleepy for a bit until your body fully breaks down the chemicals.
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