Sleep debt is a word used to describe your level of sleepiness. The more sleep debt you have, the sleepier you feel. The less sleep debt you have, the more awake and alert you feel.

Ideally, you would have no sleep debt during your work day so you feel bright and alert, and enough sleep debt at night to fall asleep quickly.

Many people don’t get enough hours sleep to repay all their sleep debt. They have to carry the sleep debt with them during the day until their next opportunity to pay it off.

But there are some people who, despite feeling sleepy during the day, just can’t fall asleep at night. Why is that? Surely people with insomnia have the highest sleep debt, so why can’t they fall asleep faster than everyone else?

This article explains the two separate processes that keep you alert and send you to sleep. Learn how to use them to your advantage to keep alert during the day and sleep deeply at night.

1. How long you have stayed awake for

One of the deciders of how sleepy you feel is how long you’ve been awake for. The technical term for this is sleep homeostasis. Homeostatic pressure builds up the longer you stay awake increasing the pressure to sleep. When you sleep the pressure is released.

How quickly you release the pressure depends on two factors:

• Sleep duration – How long you sleep for, getting your recommended hours of sleep
• Sleep quality – How much of your sleep is deep sleep

If you don’t release all the pressure it stays with you making you feel sleepy the next day. You’ll then have even more built up pressure to fall asleep.

Pressure to sleep builds up during the day and is released during the night whilst asleep.

2. Where you are in your Circadian Rhythm

So you feel sleepier the longer you stay awake. That makes sense, right? But how sleepy you feel isn’t quite that simple, as any insomniac would readily attest to.

Sleep homeostasis doesn’t explain why you can become overtired, when you just can’t fall asleep after doing an all nighter. Or why you feel exhausted at the end of your work day, but become alive in the evening.

There’s one more factor that governs sleep, your circadian rhythm, also known as the circadian alerting force. This is created by your body clock to keep you awake and stick to a good sleep schedule.

The circadian alerting force builds slowly during the day reaching its peak mid evening just before it starts to drop.

The dip at the end is crucial. It’s during that time we’re able to sleep.

Combined = The Two Process Model of Sleep

How sleepy you feel and how easy it is to fall asleep is combination of two factors:

• How long you have stayed awake for – the homeostatic pressure
• Where you are in your circadian rhythm – the circadian alerting force

Together they look like this:

The homeostatic pressure builds during the day to make you sleepy but is countered by the circadian rhythm to make you feel alert.

The dip at the end of the circadian rhythm is crucial to our sleep. It’s during this dip that we have an opportunity to fall asleep. When the circadian rhythm is no longer opposing the homeostatic pressure.

Falling asleep before this dip when the circadian rhythm is at its peak is near impossible. This is why having an early night doesn’t mean that you’ll be able to sleep any earlier.

There’s one major problem with the circadian rhythm. Whilst sleep homeostasis is relatively stable, the circadian rhythm can shift backwards and forwards in time. It might dip too soon so you fall asleep in the evening, or it might peak too late, so you can’t sleep until the early hours of the morning. This is the reason why it is so important to have a good sleep schedule. To keep the circadian rhythm firmly in place.

Your Overall Level of Alertness

When sleep homeostasis and the circadian alerting force are combined, the picture looks something like this:

There’s a slight dip in alertness during the afternoon known as the post lunch dip. Most scientists agree to its existence, but no one actually knows why its there.

By using this graph you can get an indication of how sleepy and alert you’ll feel at different times of the day.

Businesses use this graph to get the most out of their workforce. It’s called fatigue management. It’s useful for jobs where people work around the clock doing dangerous jobs that can cause harm to themselves or others. Such as fighter pilots, soldiers on the front line and overnight factory workers operating heavy machinery.

But personally, you can use this information to boost your productivity and enhance your sleep.

• Do high energy tasks when you’re alert– Creative thinking works best in the morning or in the evening a few hours before you go to bed.

• Do low energy tasks when you’re sleepy – Process your emails and perform routine tasks after lunch during the post lunch dip.

• Don’t sleep during the day if you have problems sleeping – It releases homeostatic pressure which you need enough of to fall asleep at night.

• Don’t try to fall asleep too early – This is when we have our peak alertness. Known as the forbidden zone by sleep experts because trying to sleep at this time is near impossible.

• Avoid light during the night before bed – Light prevents the circadian alerting force from dipping.

• The best time to power nap is just after lunch – You’ll be able to nap far more easily during the post lunch dip.