There is an underlying mechanism, called the sleep clock, which consists of a number of variables in your body that tell it when to feel tired, and when to feel awake. It also controls how deep you sleep, and how long you sleep.
The first, and most important part of your sleep clock is your body temperature rhythm. It’s also known as a circadian rhythm.
Contrary to what most of us are taught in grade 5 science class, our body temperature doesn't stay at a constant 98.6º Fahrenheit (37º Celsius). Our body temperature actually has a specific rhythm to it. It rises and drops as the hours of the day progress. The difference in body temperature is about 3º Fahrenheit (2º Celsius). This periodic rise and drop in body temperature tells our mind when to feel tired and when to feel more awake. As body temperature rises, we tend to feel more awake and our brain waves are usually higher. As body temperature drops, we tend to feel more lethargic, tired, and lazy - this is a big cue for our minds to lower brain waves and enter Stage 1 sleep.
There is a slight “drop” of body temperature during the mid afternoon. This is a usual mid-afternoon body temperature slump. You may have noticed, at some point in your life, that you usually feel an urge to sleep or take a nap during the afternoon. This is completely natural, and sometimes the pressure to sleep during the afternoon is as strong as the pressure at night! (Although most of us chose a drug of choice such as caffeine to combat this body slump).
Because of the demands our society puts on us, such as work, children, and social life, most of us can't sleep at this time. As we'll explore later on, nature actually intended for us to have a nap at this time - we'll talk about the science of naps in detail.
Generally, body temperature begins to rise in the early morning hours, drops sometime during the afternoon, then begins to rise until the early hours of the evening. It's at this time that we have “peak performance” body temperature, most people are most active during the early evening hours, this is where body temperature is the highest. Afterwards, body temperature drops and reaches its lowest point at around 4 am.
If your body temperature rhythm is too flat (doesn't rise or drop low enough), or if it's messed up in any other way, chances are you will experience sleep difficulties. It will be difficult for you to sleep deeply.
It's because of the body temperature rhythm that most of us feel sleepy, at precisely the same time every night. It's also why some people can wake up without an alarm clock at precisely the same time every morning. Usually, your body temperature rhythm will follow the same pattern regardless of when you fall asleep. For instance if you've been waking up at 7 AM all your life, this means your body temperature begins to rise at this time. It won't matter if you fall asleep at 11 PM, 12 AM, or 1 AM, your body temperature will rise at 7 AM, and you will feel sleepy at the same time you always did the next day. Unless you take the proper actions to optimize your body temperature, it will usually return to the same pattern.
This is the primary reason why jet lag happens. When you travel really quickly across several time zones, your body may be in a different time zone, but your temperature rhythm is still following the pattern it did before! So if you normally live in Florida, and you take a flight to California; if it's 8 PM in California, your body will still think it's 11 PM, based on your temperature rhythm. As you see, your temperature rhythm really acts as an internal “clock”. Your body temperature rhythm can adjust to a new time zone, or a new sleeping pattern, and this may take from a few days to up to several weeks! This is why trans-continental jet lag is so severe for some people.
Your body temperature rhythm is perhaps the most important concept to grasp about your inner sleep clock. It has a huge impact on how you sleep, and how you function during the day. So what affects your body temperature rhythm? And how could someone possibly “damage” their body temperature rhythm?