The circadian rhythm
As you know, there are 24 hours in a day. During all of these hours, there is biological activity going on inside you. This activity is also known as circadian rhythm. The most apparent circadian rhythm is your sleep-wake cycle. Just like the alarm next to your bed, which you need to wake up at a particular time so you won't be late for work, your body has an alarm clock as well. It is like an internal clock, which wakes us up and regulates when to activate systems in our body. Not everybody has the same sleep-wake rhythm, which is why we have morning-people and evening-people. But, the figure below will give you an example of biological and physical events during the 24-hour biorhythm.
Why is this important? Simply put, your body is primed for different types of activity during the day. And practically everything you do benefits from a stable and synchronized circadian rhythm. For example, you have planned to work overtime tomorrow, so you won't be able to join your standard 18:30 CrossFit class. You decide to do your workout at 07:00 instead. Although there is nothing wrong with exercising in the morning to start your day right, since you desynchronize your routine, you probably won't hit your usual numbers, times or weights.
Consistency is key
Your body auto regulates your circadian rhythm quite well on 'time givers'. Primary time givers are light, physical/mental activity and food. As long as your lifestyle is consistent, your circadian rhythm will synchronize, improving your sleep quality.
There is a window
Between commuting, work, family, training and socializing, many obstacles can keep you from being consistent. That doesn't mean you can't try to be as consistent as possible. When consistency is difficult, use a 2-hour window for your meals and training. So if you usually exercise or eat a meal around noon, but for some reason, that is not doable today, then any time between 11:00h and 13:00h would be acceptable.
Sleep is more sensitive to circadian disturbances, so i.o. a 2-hour window, it's advisable to keep your bedtime within a 1-hour window. This will significantly improve your sleep quality.
Let there be (no) light
The other major, and arguably the most critical, time giver is light. Light gives a signal to our brain that wakes us up. You've probably heard of 'blue light' leading to suppression of melatonin production, a hormone contributing to sleep quality. Much is still unclear about blue light and decreasing sleep quality. But simply said, you should turn on bright lights when you wake up and turn them down or even off when you want to sleep. This is also why some people use a wake-up light. If you haven't tried it yet, give it a try. It's not that expensive, and why wake up to an annoying alarm when you can wake up to the light.
Morning larks and night owls
The circadian rhythm is different for every individual. Some people like to do their workout at 07:00, which for some people sounds like madness. Some people can only train after 20:00, which is way too hard for others. You may know what's the best time for you to train, but if you want to check if you are a morning lark or a night owl, you have to sleep without an alarm for a couple of days. This way, you can fall asleep and wake up on your internal clock. Doing so for a couple of days will give you your approximate sleep time. Dividing this by 2 gives you your mid-sleep time. If this mid-sleep time is before 2 a.m., you are a morning lark, and if this is after 4 a.m., you are a night owl. It is estimated that 25% of all people are morning larks and 25% night owls. The remainder falls in between both. Morning larks are advantageous to night owls who constantly live in so-called social jetlag because of our 9-5 society. That's why night owls might have to learn to nap to flourish.
One of the best things in life are naps, snooze away in the sun or the passenger's seat…. Although no amount of naps can make up for structurally getting too little sleep at night, they can make you feel better and help you focus. If you want to take a nap, make sure to put on an alarm anywhere between 20-30 minutes. After 30 minutes, you will get into your deep sleep, causing you to wake up feeling like you got hit by a truck. So put on a timer and lay down. If you didn't fall asleep, you still took some time for yourself, which is fine.