Self Sabotaging Sleepytime Stupidity

Our bodies need sleep, they know that. Our brains get more tired towards the end of the day and we yearn to finally call it quits and go to bed. So why are nightmares? Why are scary thoughts before sleepy time? 
    I shall elaborate. 
Though this isn’t whilst I’m sleeping, it’s the before times of getting there. I shower right before bed and so many damn times I think “what if there’s something in here with me?” be it a person or a demonic creature, be it in the tub itself or just the bathroom, my brain gives me that to think about. It does the same thing as I lay in bed, letting my body succumb to sleep. I suddenly wonder if there’s someone outside my door waiting to break in or something standing in the corner of my room or a figure right by my bed but I can’t see because I’m facing the wall. My brain convinces itself that it senses we are not alone when I’m at my most vulnerable. It throws out all these unnerving thoughts and ideas and haunts me when I’m just trying to pass out for the day. 
Then the nightmares come. I spend my entire waking life stressed and on edge and anxious and exhausted. I do not want the same for my sleeping hours. I need balance. Even the dreams of working a whole day after actually working a whole day pisses me off because I don’t want to think about that now. Let alone the terrifying images and stories that are dealt with subconsciously like watching my family die or running from something I can’t actually escape or the world around me warping onto something claustrophobic and dark and confusing. 
To spend the night seeing unsettling things seems wasteful and to spend the time before sleeping imagining unsettling things seems unnecessary. As I said, our bodies know that sleep is important so it weirds me out that they would self sabotage that for no apparent reason. I just wake up more tired and full of negative feelings and emotions which doesn’t help anything. Who came up with this? Why did we evolve like this? What is the purpose of this? I need a lot of sleep to function and I already lose a lot of that by taking forever to finally fall asleep and by waking up again due to either a full bladder or chronic pain. I cannot afford to lose more by freaking myself out a half hour before going to bed, to the moment I’m trying to sleep, to my subconscious after the fact. 
Let life continue being my own personal nightmare, but at least give me the nights to dream. I’m so tired and I think too much and there’s no one here with me so I’m left to fill in faces and appearances for all the sounds I hear, oh the odd noises that occur. I try to drown it all out with soundscapes and the light that emanates from my wee air cooler but alas, this is where my vivid imagination has its faults. Where I can see good things clearly before me if I wish it, so can I with the bad even when I don’t wish it, and unfortunately the bad often invades my sight when it’s dark and I’m lonely.