"How refreshing it is after a good night's sleep!"
- TKV Desikachar 
We all know this from our own experiences. And we know how difficult it is to get a good night's slept and this is probably due to the way our modern environment, society and economic system works.
Think about how much harder it is these days to say when work actually starts and finishes. For many of us it is no longer the case that we start a 8 or 9 in the morning and finish 8 or 9 hours later. Smart devices, linked with feelings of insecurities and or the desire to advance one's career, have blurred the lines between work and play, and work and home life.
These days it is easy to start looking at work emails when we wake up, as we travel to and from work as well as last thing at night.
If you think about it work doesn't satisfy all our needs. In fact, it can't. We need to have time for family, friends, community and for to ourselves too.
These days, fitting in a bit of time at the gym or on the yoga mat and fun with family and friends is most likely achieved by cutting down on the amount of sleep we allow ourselves. We deceive ourselves into thinking that a nightcap before bed will help us to sleep and that regular cups of coffee will get us through the day. Self-medicating on evening sedatives (alcohol) and on morning and afternoon caffeine (coffee, tea, chocolate) is not an uncommon recipe applied to allow life to go on normally without reducing the quality of our thoughts, feelings and actions.
It turns out that this is not true. Sleep is important.
If you want to be successful at yoga - at anything, for that matter - don't negotiate away the 8.5 hours of sleep opportunity we need each and every night.
T Krishnamacharya explained, in his introduction to Yoga Makaranda, that
if we do not get the stipulated hours of sleep, "then our physical and psychological nature (svarūpa) will totally be destroyed" .
If you have read Matthew Walker's 2017 book Why We Sleep: the New Science of Sleep and Dreams, you may have been shocked to discover that modern science is also telling us that undervaluing the importance of sleep has serious consequences on the length and quality of life. It turns out that:
“the shorter your sleep, the shorter your life. The leading causes of disease and death in developed nations—diseases that are crippling health-care systems, such as heart disease, obesity, dementia, diabetes, and cancer—all have recognized causal links to a lack of sleep.” 
I've learnt a lot about the importance of sleep from this book. Most importantly, we must sleep between 7 and 9 hours each and every night, without exception, to avoid the harm sleep deprivation causes us.
With less than 7 hours of sleep you can objectively show under performance in ALL systems of the human body. To paraphrase, Matthew Walker:
Sleep is the very foundation upon which we build our lives.
If we want to live long, happy, healthy and fulfilled lives we MUST give ourselves the opportunity of getting 8.5 hours of sleep each and every night at the same time each day.
T Krishnamarchya repeated the same wisdom almost 100 years ago: "From sleep, one gets physical health and agility. Further, it is not an exaggeration to say that both the body and the mind get rejuvenated by sleep." 
The Yogasūtra of Patañjali - my turn-to text for many questions on yoga and its practice - has an interesting sūtra [YS I.38]:
which translates roughly as:
Mental stability can arise from reflecting on the experiences that come from dreams [svapna] and deep sleep [nidrā].
It comes in the section that presents a range of methods that calm and realign or stabilise the functions of the mind.
There are many esoteric points to be made about this sūtra, but it seems to me that the very first experience or knowledge of dreaming and deep sleep we have is that of having good nights and bad ones. When we sleep badly our minds are far from stable the following day. We all know how rubbish we are when we don't get enough sleep. The more sleep deprived we are the more disturbed are minds become.
I feel that our modern digital culture disconnects us from what it means to be deeply human and replaced it with something that glitters like gold but isn't. The greater connectivity to knowledge, its almost indistinguishable opposite: fake news, and to distant people and communities are mixed blessings that can take us away from what really matters in life.
If Shakespeare is someone to go by, then we know that our ancestors knew the value of a good night's sleep. Look what happened to Macbeth.
Shakespeare points out in six lines the benefits of sleep as "chief nourisher in life's feast", "Balm of hurt minds" and how the lack of sleep contributes to Macbeth's mental deterioration:
Me thought I heard a voice cry ‘Sleep no more!
Macbeth does murder sleep,’ the innocent sleep,
Sleep that knits up the ravell’d sleave of care,
The death of each day’s life, sore labour’s bath,
Balm of hurt minds, great nature’s second course,
Chief nourisher in life’s feast.
- Macbeth (2.2.46-51)
Again, modern science is showing the importance of sleep and pointing out what happens when you don't get enough.
In the short video that follows, Matthew Walker lists some of the awful results routine lack of sleep has on our health span, which can be shorter than our life span.
Lack of sleep:
will actually prevent your brain from being able to make new memories
will lead to an increase in your risk of going on to develop dementia in later life
will age you by almost a decade in terms of that aspect of virility and wellness.
impacts your immune system. So after just one night of four to five hours of sleep, there is a 70% reduction in critical anticancer-fighting immune cells called natural killer cells. And that's the reason that we know that short sleep duration predicts your risk for developing numerous forms of cancer
impacts your cardiovascular system because it is during deep sleep at night that nourishes your heart
gives a 200% increased risk of having a fatal heart attack or stroke in your lifetime
It seems that we can be awake for about 16 hours before needing about eight hours of sleep to repair the damage of wakefulness.
"Wakefulness", says Matthew Walker, "is essentially is low-level brain damage."
In this context, it may be reassuring to know that taking up and maintaining a suitable daily yoga practice, which often is simple and easy to do, improves sleep duration and quality. In my experience, improvements in sleep is one of the key benefits reported by people taking up yoga. From a wellness point of view, giving sleep its proper value and priority should be the cornerstone of any strategy to improve quality of life.
Notes & References:
 TKV Desikachar, 1995, The Heart of Yoga, p.161
 T Krishnamacharya, 1934. Quotation is from p.44 of the Yoga Makaranda: the nectar of yoga. Originally published in the Kannada language in 1934. Revised English translation by TKV DEsikachar with the assistance of ER Ramaswamy Iyengar, 2011
 Mathew Walker, 2017, Why We Sleep: The New Science of Sleep and Dreams, p. 164
Photo by Paul Stollery on Unsplash