Afternoon naps are a double-edged sword. Sometimes you can wake up from them feeling like you're
on top of the world and alert, motivated and happy. Other times, you feel like you're barely
clinging on for dear life and feel even more fatigued than before the nap. This grogginess
you feel after some naps is called "sleep inertia". Sleep inertia reduces our cognitive
performance and makes us miserable and unproductive. I'm a doctor from Australia and my goal in
this video is to share what I've learnt about napping and show you how to optimise your
naps to avoid sleep inertia. First, to understand how to prevent sleep
inertia, we first need to understand how it works. When we sleep, a lot of complex things
happen in our bodies but I'll try simplify it here to the bare minimum of what you need
to know. So there are two types of sleep: Non-REM sleep
and REM sleep. REM stands for rapid eye movement and we'll talk more about it in a second.
Non-REM sleep is divided up into 3 stages. Stage 1 is where your body starts to relax
and your heartbeat, breathing, and eye movements slow down. If you're woken up from stage 1,
it might feel like you never even slept at all. Doctors can identify the different stages
of sleep by looking at your brain waves through an EEG. When you're awake and relaxed, your
brain produces alpha waves which look like this. Once we reach stage 1 of sleep, we start
to produce theta waves which are lower frequency than alpha waves, meaning your brain activity
is slowing down. Stage 1 occurs only for a few minutes before we move onto stage 2
In stage 2 of sleep, your heartbeat, breathing and eye movements continue to slow down, your
muscles relax and your body temperature drops. We still have theta waves but we also have these
brief periods of high electrical activity called K-complexes and sleep spindles. Stage
1 and 2 are still considered light sleep. However, stage 3 marks the start of what we
call deep sleep. This is where your heartbeat, breathing and muscle activity reaches the
lowest levels. Your EEG shows delta waves which are very low frequency waves, which
is why this stage of sleep is also called slow wave sleep. Deep sleep is important for
memory consolidation where short term memories are converted to long term memories.
After stage 3, on a normal night of sleep, you'll usually go back into stage 2 before
going to REM sleep. In REM sleep, your eyes which move rapidly from side to side behind
your closed eyelids. The brain waves become desynchronised and faster, similar to when
you're awake. This stage is when you dream vividly. Your body also becomes paralysed
with the exception of your eye muscles and breathing muscles so that you don't act out
your dreams. This whole cycle takes on average 90 minutes
and occurs 4–6 times in a good night's sleep. Sleep inertia occurs when you're woken up
from slow wave sleep, that is stage 3 of non-REM sleep. It's hard for your brain to transition
from a period low activity and force it to get back into gear straight away and this
transition lag is what makes us feel groggy. The question is what is the optimum length
of time we can nap where we're still getting the benefits of a nap but we don't feel groggy
when we wake up? A series of experiments actually found the answer to this question but I feel
like not enough people are aware of these results, which is why I'm making this video.
In the first scientific study, Tietzel and Lack allowed participants to sleep for 10
minutes or 30 minutes. They found that the 30 minute group and 10 minute group both had
the increased alertness and cognitive performance after the nap. So they hypothesised that it's
not depth or length of sleep that matters but sleep onset which causes the benefits
of sleep. In other words, just reaching stage 1 of sleep could lead to increased alertness
and cognitive performance. So the same authors did a second study where they got participants
to sleep for 30 seconds, 90 seconds or 10 minutes. Just keep in mind that the scientists
used EEG monitors to determine when they fell asleep, so the actual time they spent in bed
was likely much longer. What they found was that the 10 minute nap led to increased alertness
and cognitive performance but the 30- and 90-second naps had no benefits. So clearly
just reaching stage 1 isn't enough, you have to go through stage 1 and dabble into stage
2 before you got any benefits from the nap. Tietzel and Lack performed a third study to
answer the all-important question of what the most optimal napping duration is to minimise
sleep inertia. They had participants nap for 5-, 10-, 20- or 30-minutes. They found that
the 5 minute naps produced no cognitive benefits, nor any significant sleep inertia. The 20-minute
nap produced sleep inertia for 95 minutes and 30-minute nap produced sleep inertia for
155 minutes. However, after the sleep inertia dissipated there were still improvements in
alertness and cognitive performance compared to before the nap. What's interesting is that
the 10-minute nap produced even better improvements in alertness and cognitive performance than
the other nap lengths, but had almost no sleep inertia. After 3 hours though, all the groups
went back to their baseline. Later studies confirmed that 10-20 minute naps are the best
because after this we start to go into slow wave sleep, which as we know from before,
leads to sleep inertia. 30 minute naps have on average 7.5 time more slow waves than 10
minute naps, showing you that even snoozing your alarm for 20 minutes can have a huge
effect on how you feel afterwards. There is one important caveat however. Several
external factors influence your ideal nap time, such as your genetic makeup, your sleep
deprivation and where you are in your circadian rhythm. For example, if you're sleep deprived
or you're in a dip in your circadian rhythm like in the mid-afternoon, then you might
reach slow wave sleep quicker than usual so your nap length will need to be adjusted accordingly.
Unfortunately as much as you try, you might still feel a bit groggy after a 10-20 minute
nap. Luckily, the scientific literature has found a foolproof way to eliminate sleep inertia
in almost anyone who needs a quick afternoon snooze. It's the coffee nap and you may of
heard of it before. The idea is to drink coffee and then take 20 minute nap before the caffeine
kicks in. When you wake up, it should coincide with the onset of caffeine's action on your
brain, causing you to reap all the cognitive benefits of a nap without the grogginess.
Okay so when I first read this research, I had a lot of questions pop up in my head which
I have since found the answer to. So let me share with you what I found.
Question 1: How do we account for sleep latency? Sleep latency is the time it takes us to fall
asleep. In scientific studies, they use EEG monitors to measure when participants reach
that first stage of sleep and then start counting from there. So we know that 10-20 minutes
of sleep is ideal but how long should you set my alarm for? For me, I can set my alarm
for 20 minutes, which means I fall asleep in about 5 minutes and get about 15 minutes
of sleep. For you that might be a bit longer or shorter, so you'd have to experiment with
what works for you. Question 2: What about if we just complete
a full sleep cycle which is about 90 minutes? Well the literature isn't fully clear on this
but Lumley et al. found that a 120-min nap which predominantly had REM did not improve
cognitive performance over a 60-min nap with no REM. However, in the long term, REM sleep
benefits individuals in other way like improving learning and certain types of memory, so make
sure you sleep properly at nighttime. Question 3: Is it better to avoid naps and
sleep earlier at night? Well, I used to avoid afternoon naps and force myself to stay awake
because I thought it would sap time from my day but then I realised a short 10-20 min
nap is much better for my productivity in the afternoon when I'm feeling tired. The
literature supports this and shows that naps have several short term benefits like increased
alertness and concentration, improved mood, reduced anxiety and improved reaction times
and memory. Question 4: Don't afternoon naps impact your
sleep at night? Yes if you nap too close to bedtime. The Tietzel and Lack study found
no change in sleep latency beyond 3 hours after a napping, so they suggested that we
nap a minimum of 3 hours before bedtime. However, their study was done in sleep deprived individuals
so for most people it's probably safer to nap as early as possible to avoid delaying
your sleep at night. Also if you're doing a coffee nap, the caffeine will stay in your
system and impact your sleep for anywhere between 5-8 hours after drinking it.
So in summary, if you're feeling tired throughout the day, taking a nap between 10-20 minutes
is the sweet spot. This might mean setting the alarm for 20-30 minutes depending on how
long it takes you to fall asleep. Any less and you won't receive the benefits of napping
like improved alertness, cognitive performance or memory. Any more and you'll feel worse
off than before because your brain will enter deep sleep and experience sleep inertia when
waking up. Just keep in mind the time it takes to fall asleep and the time of onset of slow
wave sleep is slightly different for everyone, so experiment with what works for you.
If you found this video interesting you'll like my last video which was about why sleeping
late and waking up late doesn't mean your necessarily lazy. Click the link on the cards
or in the description to watch that. And if you learnt anything in this video please consider
subscribing. Cheers.