When it comes to getting our babies to fall asleep we're all aware that being awake before a nap or a night sleep is a big part of that.

On some level, a lot of us understand the time awake builds sleep pressure, and society and social media has pumped into us the message that too much awake time is bad as this makes our babies over tired.

But what about not enough awake time? What happens when our babies are awake, that then encourages them to sleep?

A big chunk of what happens when our babies are awake that contributes to this sleep pressure and need to sleep, is the build up of a homeostatic modulator called adenosine. 

Building sleep pressure

• Think of adenosine as a “tiredness meter.”
• While you’re awake, your brain cells burn energy (using ATP). Each time ATP is broken down for fuel, little bits of adenosine are released.
• The longer you stay awake, the more adenosine piles up in the brain, especially in areas that regulate alertness.
• As levels climb, adenosine binds to special receptors, dampening brain activity and creating that heavy, sleepy feeling often called sleep pressure.

Helping your baby fall asleep

• Once the pressure is high enough, adenosine tips the balance so your baby’s brain shifts from wake-promoting circuits to sleep-promoting circuits.
• It works a bit like filling a balloon: once the balloon is stretched to its limit, it has to let the air out. Similarly, when adenosine reaches a threshold, your body gives in to sleep.

Keeping your baby asleep

• Adenosine continues acting on its receptors during the early part of the night, helping to keep brain activity quiet and stable.
• This is especially important for deep sleep (slow-wave sleep) in the first half of the night, when your baby’s brain does a lot of repair and recovery work.
• As your baby sleeps, their body gradually clears adenosine from the brain. By the second half of the night, levels are lower, which is one reason why sleep becomes lighter and dreams (REM sleep) are more common, and more wake ups are seen. (think early wake issues!) 

Why you wake up refreshed

• By morning, most of the adenosine has been broken down and flushed away, meaning that “sleep pressure” has been released.
• This clearing is partly why a good night’s sleep makes your baby feel alert and happy and their temperament is great. Whereas staying up too late means you still have high adenosine and feel groggy.

Food for thought

• Caffeine before bed: If you drink coffee late in the day, caffeine blocks adenosine receptors. The adenosine is still there, but the brain can’t “see” it — so you don’t feel sleepy. When the caffeine wears off, all that built-up adenosine suddenly binds at once, which is why you might feel a “crash.”
• All-nighter: If your baby stays up a lot at night, adenosine builds to very high levels. The next day, even if you try to keep your baby awake, they’ll feel overwhelming pressure to nap. We try to limit their naps in this case, knowing the extra sleep pressure will help us the following night!

If your baby gets over tired….

If you push your baby past their natural sleep window:

1. Adenosine is high → their brain is screaming for rest.
2. But their body responds by releasing stress hormones, especially cortisol and adrenaline, to keep them awake and alert.

• This is part of the “second wind” hyperactivity you sometimes see when your baby is exhausted but suddenly wide awake.

The problem

• Cortisol and adrenaline counteract adenosine’s sleepiness signal by revving up your baby’s brain and body.
• That’s why when they’re overtired, they often act “wired but tired” — their body wants to sleep, but stress chemistry is keeping them awake.
• This can make it harder to fall asleep, even though adenosine levels are very high.
• Sleep, when it comes, may be lighter and more broken, because cortisol interferes with the deep restorative stages that adenosine usually supports. We see frequent night wake ups, and a lack of self settling.

In babies and children the problem is even bigger

Adenosine builds quickly because babies’ brains use huge amounts of energy for growth and learning, so adenosine builds up faster than in adults. This means they hit their “sleep pressure threshold” sooner. This is where awake windows come into play. As your baby ages week by week, they build less adenosine and need less sleep, or need longer awake to build enough adenosine to need a nap!

If they’re put to bed on time, after the right awake window, then adenosine is high → they drift off easily, and probably stay asleep for a good length of time. Cortisol is still low in the evening therefore nothing is blocking sleep pressure.

However, if they’re kept awake too long past their natural sleep window, they become (overtired). Adenosine is still very high, but the body starts releasing cortisol and adrenaline to keep them going. This creates the “second wind” — suddenly the tired baby looks hyper, wriggly, or cranky instead of sleepy.

Trouble is looming…

• Harder to fall asleep: Cortisol overrides the drowsy signal from adenosine, so the baby fights sleep even though they’re exhausted.
• More night waking: High cortisol makes sleep lighter and more broken. That’s why overtired babies often wake more during the night.
• Early rising: Cortisol naturally rises toward morning. If levels are already high from being overtired, it can push them to wake up too early (e.g. 5am).

Emma Purdue

Emma is the owner and founder of Baby Sleep Consultant, she is a certified infant and child sleep consultant, Happiest Baby on the Block educator, has a Bachelor of Science, and Diploma in Education.

Emma is a mother to 3 children, and loves writing when she isn't working with tired clients and cheering on her team helping thousands of mums just like you.

If you're having trouble striking the balance with awake windows, overtiredness or under-tiredness, lets chat! Send us your details and a team member will be in touch...