If you're reading this blog, chances are you already know much of this, but I'd like to give some thoughts on the issue of sleep-related neurotransmitters, vitamin D, and their effect on sleep- which in turns affects nutrition, health and function. The key to all of these elements working to successfully facilitate the process of sleep is "synergy." Exposure to the stimuli that create such synergy is disrupted by things like culture and technology.
Early man (or "Grok") may have been a cave-dweller, but managed to get plenty of sunlight during waking hours. Typically, Grok would have woken up with early morning "gentle" sunlight, and gone about his day. Hunting and gathering food (including scavenging) were activities of focus for most early man (as well as modern hunter-gatherers). Spending hours under the sun garnered large doses of Vitamin D during Grok's quest for food.
Exercise would be a natural component of such activities, as sprinting would be required to hunt game. Lifting heavy things would be a component of both hunting and gathering as food items (carcass and plant) would need to be hauled back home for preparation and consumption. Typically, both hunting and gathering required long hours of walking at a slow pace.
At the end of the day, dusk would induce sleep, and Grok would have only the company of his fellow tribesman and perhaps a (low lumen) fire to compete with the urge to sleep.
This was the algorithm by which humans evolved the biological mechanisms for survival. Understanding this is helpful in explaining why so many people are sleep-deprived today.
Modern man, on the other hand, is obsessed with avoiding sunlight slathering copious amounts of sunscreen on his skin in an effort to avoid getting skin cancer. Instead of hunting and gathering, we now spend hours under fluorescents in offices or cubicles staring at computer screens and sitting down without any exercise to speak of. This is, in the literal sense, the exact opposite environment in which we evolved.
Lights out and morning light
Light sensitive hormones and neurotransmitters like serotonin, melatonin, and dopamine, are secreted relative to the rising and setting of the sun. It's this morning light that sets our clocks to the circadian cycle. Here's the catch: humans are now constantly bombarded by artificial light throwing off the timing of sleep rhythms. If you're reading this blog at night, you too are guilty of subjecting yourself to an artificial light source. Video screens are responsible for disrupting the sleep of millions of people all across the globe. Don't subject yourself to this! Staying up later and waking up sooner is the downfall of many. Turn off the TV, turn off your computer! Facebook can wait till tomorrow. (preferably when you're at work and able to do it on your boss's dollar) I can hear my wife now: "Doctor heal thyself!"
Now, I ALWAYS recommend natural sources over supplementation! I want to make that clear. But in light of a deficiency, people who are deranged in any neurological or metabolic pathway can benefit from short-term supplementation in order to "right the ship," with the end goal being a tapering off of supplements and reliance on natural food or environmental sources.
Melatonin is a neurotransmitter secreted by the pineal gland during the dark hours of night. This, assuming you have not down-regulated your melatonin receptors, should be enough to cause the feeling of sleepiness. Melatonin acts in synergy (there's that word again) with decreased sunlight to produce the feeling of sleepiness. However, all that simply means is that one has a propensity to fall asleep at sundown, but doesn't necessarily imply that sleep at sundown will be a guarantee. I mention this because often I talk to people that try melatonin for a while, and then report back with how unsuccessful the supplements were in helping them sleep. Such incidents are not surprising in light of the fact that they work in an office all day, don't get any mid-day sun, and then watch TV and hang out in a well-lit room, with all sorts of stimulants and activities during the late evening. Think of when the sun goes down. That's when melatonin would naturally start being secreted. Turn your lights down or off at that time!
Caution: Melatonin is a hormone, and can be every bit as dangerous as many drugs. It CAN harm you with over dosing, and over using. As Mark Sisson says in a post here, he's not a fan of taking melatonin on a regular basis, but is okay with using it to "reset" your diurnal clock when traveling. My take is that if you're deranged, you need to reset your clock regardless of whether or not you're traveling, and using the supplement once or twice is not going to get the job done in people with impaired sleep cycles. Use it for 3-5 days max until you're able to get to sleep and then use it only on an "as needed" basis. Eventually, you want to get to the point to where you use melatonin supplements VERY rarely. (For example, I've used melatonin once this year) The worry is that down-regulation of receptor sites or reduced secretion of natural melatonin can occur with over-supplementation, and I concur with that angle.
Vitamin D is a fat-soluble vitamin that is only produced by the body in the presence of sunlight and UV exposure. There is some controversy as to the efficacy of vitamin D supplementation, with some claiming that supplements are unsafe. Also, the efficacy of supplemented vitamin D is relative to the fat composition of the diet, in terms of PUFA ratios (Omega 3s and 6s) as well SFA and MUFA intake.  Frankly, I'm not convinced of the evidence for moderate supplementation being unsafe, but again for peace of mind it would make much more sense to simply get 15-30 minutes of mid-day sun rather than taking a pill. However, for a large part of the population, mid-day sun exposure is not possible, when either climate is an issue, or they (more commonly) are office dwellers.
Personally, at work I attempt to get out once a day for lunch. Even with clothes on, it's a better option. Plus, there's a synergistic psychological response to being out in the sun that, along with vitamin D, helps regulate sleep cycles. If you choose to supplement (as I do when I can't get out during a shift) then I recommend doing so during the hours of 10am to 3pm to mimic when such exposure to D would typically occur. 15-30 minutes of mid-day sun will garner the average person about 15,000-40,000 IU of naturally produced vitamin D. However, I'd consume no more than 2,000 IU if supplementing as the typically manufactures use PUFA (often oxidized) as a vehicle for getting the vitamin D in to your bloodstream.
Another neurotransmitter that works synergistically with vitamin D, and UV exposure is serotonin. You may be familiar with serotonin as a mood hormone. Selective Serotonin Reuptake Inhibitors (SSRIs) such as Lexapro or Celexa are drugs used to enhance mood, in the clinically depressed. They seek to inhibit reuptake of serotonin, thus flooding the synapses with the hormone inducing an enhanced mood. Often people placed on SSRIs are heavy sleepers as a result.
Luckily, there is a natural way to increase serotonin levels and yet again, the sun is the source. Lower levels of sun exposure are greatly associated with sleep disturbance. [2, 3]
"The study reports that levels of serotonin in the brains of participants increased in direct relationship to their exposure to sunlight. Catheters placed in the internal jugular veins of participants allowed assessments to be done as these people were exposed to varying degrees of sunlight. The study found that “the rate of production of serotonin by the brain was directly related to the prevailing duration of bright sunlight, and rose rapidly with increased luminosity." [3,4]"
Of course Grok might have had occasion to stay up late with stimulating activities like having fun, singing, dancing, watching out for predatory animals, intruders, inclement weather, etc.
As some of you may recall from A&P, epinephrine (aka Adrenaline) competes with melatonin receptor sites, reducing the sensation of sleepiness. Workout at 9pm and then try to go to bed at 10pm for an example of this. I don't care how much melatonin one consumes, if you clean and snatch your max just before hitting the sack, you're doomed to a restless night's sleep at best.
But, more often than modern man, just by virtue of having less epi-stimulating activities, as well as less artificial light, sleep disturbances were most likely rare. (those with such disturbances were probably unfit and selected out.)
The Grok meme can be useful in terms of its simplicity to explain many "whys" but can often point one in the wrong direction as well.
Speaking of epinephrine, fight or flight is a protective mechanism that humans evolved to cope with survival situations, typically in life or death situations. In today's day and age, we are faced with many "Stressful" situations which cause the hypothalamus and anterior pituitary gland to secrete cortisol. Cortisol is also regulated by the sleep cycle in response to light. "Information about the light-dark cycle is transmitted from the retina to the paired suprachiasmatic nuclei in the hypothalamus."
This is significant because low-level chronic stress (physical or psychological) increases serum levels of cortisol which is typically moderated by rising and falling of the sun. But if stress is sustained in to the evening and night, cortisol will continue to enter the blood stream, throwing off sleep patterns and reducing REM time. 
In the Paleo world, over-training is a common source for chronic cortisol secretion. To remedy this, ensure 2-3 rest days and avoid chronic cardio.
Avoiding stress, although impossible to eliminate, is key to reducing chronic cortisol syndrome. Play, love, and laugh! Increase your focus on hobbies, sports, mediation, and hiking. Mix in a beer (gasp!). Spend time with your family. And for the love of Pete, balance your work life with your personal life! Time is precious, and the pursuit of happiness doesn't always include a larger paycheck or a promotion.
So here are my recommendations:
1. Get as much mid-day sun as you can without burning yourself. (The time will vary from person to person based of skin pigment and genetics) If you can't get outside, take a vitamin D supplement. At work, take your breaks and walk a bit outside.
2. At sundown, turn all lights out, and minimize your exposure to artificial light sources like TVs or computers. In your bedroom, eliminate all night lights in your room, even covering up any alarm clock displays.
4. Eliminate all stimulating activities past sundown, such as dancing, exercising, walking, etc.
5. If necessary, take a melatonin supplement about 30 minutes before your desired bedtime. Do this regularly for a week, and then taper down to using melatonin only when you have trouble going to sleep (i.e. when you're up for over an hour in bed). Ensure your photo-exposure is minimized, otherwise you render this useless.
6. Get regular exercise during the day time (office dwellers, walk outdoors on breaks, and use stairs. If possible use a standing desk).
NOTE: citations are linked throughout the article.