Chapter 1

An introduction to how and why sleep is important and the long term consequence of not getting enough sleep - between 7 to 9 hours daily.
The author goes on to describe the various benefits of sleep and also how science has been ignorant on the benefits of sleep until the recent past.
He also says good sleep is critical and forms a “Health Trinity” - along with balanced diet and exercise - which is crucial for a healthy long term life.

Chapter 2

Two factors determine when you want to sleep and when you want to be awake:
- Circadian Rhythm
- Sleep Pressure
The circadian rhythm and the sleep pressure work in tandem with each other forming a cycle that enables us to sleep and wake regularly.
This is also why, when you pull an all-nighter, you still feel fresh early in the morning despite the complete lack of sleep the previous night. Circadian rhythm starts kicking in early morning and that rhythm is oblivious to whether you’ve slept the night before or not.

The time we wake up, the time we sleep, our body temperature and a lot of other factors
It is also known as the body clock and it resets every day
It’s primarily dependent on sunlight and it lasts more than 24 hours and not precisely 24 hours. All living beings have their own circadian rhythm.
Other factors which might influence the circadian rhythm are food, exercise and temperature fluctuations. Social interactions are also a factor.
Any such signal which helps in resetting the body clock is called Zeitgeber (German for synchroniser)
The circadian rhythm is governed by the Suprachiasmatic nucleus.
The circadian rhythm peaks around midday and gradually reduces (along with the body temperature) as the day draws to a near
The rhythm is unique to every individual. Some might peak early and some might peak late.
The former are called morning persons where their wakefulness kicks in early and the latter are evening persons.
The suprachiasmatic nucleus conveys day and night time to the rest of the body through a messenger called as Melatonin. It’s a hormone that’s released towards dusk and which is pumped through blood to the rest of the body signalling that’s its dark and time to sleep. Melatonin DOES NOT cause sleep however.

A chemical called adenosine builds up in your brain through the day. This registers the amount of time you’ve been awake in the day.
The more it builds up, the “sleepier” you get aka build sleep pressure. It’s the second factor that influences your body clock after circadian rhythm.
You can artificially mute the sleep signal by consuming a chemical that makes you alert and awake- caffeine.
Caffeine works by blocking out receptors of adenosine. It acts like a mask and thereby prevent the sleep signal to the brain.
This will meanwhile make you alert and active despite the high levels of adenosine already in your system.
The peak of caffeine intake happens after 30 minutes of oral administration. The problem however, is that it persists for a long time after being administered.
The “half-life” of caffeine is about 5-7 hours meaning 50% of the caffeine administered still exists after 7 hours. A cup of coffee late afternoon blocks you from having a good night’s sleep. Such is the influence of caffeine on your body.