Most days, I carve out 10-30 minutes for a midday meditation to reset the mind and optimize cognitive performance. The brain represents just 2% of body weight, but accounts for about 20% of the body's energy needs and consumption. 25% of the brain's energy consumption is used for housekeeping like maintenance of cell walls; 75% is used for information processing, such as computing and transmitting neural signals.
Attention and energy fluctuate as the day progresses. The main energy currency of our brain cells is a molecule called adenosine triphosphate (or ATP), which our body makes from sugar and oxygen. Increased brain activity triggers an increased supply of oxygen-rich blood. And increased mental load (such as performing mental arithmetic, reasoning, or multitasking) is linked to increased oxygen consumption.
We experience peaks and troughs in cognitive performance. When depleted, we experience cognitive fatigue. Cognitive fatigue has a neuro-metabolic correlate. Excessive thinking leads to high glutamate concentration and glutamate/glutamine diffusion in the lateral prefrontal cortex (Wiehler, 2022). Glutamate is a neurotransmitter. It is excitatory. At the experiential level, exerting cognitive control requires more effort and feels like exhaustion. At the molecular level, potential toxins are recycled and circulate if they aren't cleared out. Excessive concentrations of glutamate are associated with hippocampal volume loss, and in extreme cases, psychosis (Mancini, 2023).
One of the techniques I use to recalibrate the mind is yoga nidra. Yoga nidra (or yoga sleep) is a state of deep, non-REM rest. The brain falls into a delta wave state (<4 hz), consistent with deep, non-REM sleep, yet the practitioner remains conscious. There are 4 levels to the practice (Parker, 2013). Level 1 represents a state of deep relaxation. The brain is in an alpha state (8-13 hz). It may drop to a theta state (4-8 hz), Level 2, during the deepener practice. In Level 3, we approach a state yogis call abhava-prataya; thought ceases, but awareness remains. We experience a deep state of rest, but remain aware of our surroundings. At Level 4, a practitioner remains in a simultaneous state of sleep and conscious awareness.
Below is a real-time EEG graph of my brain's activity during a yoga nidra practice. I drop to an alpha state for about 14 minutes, then cycle all the way down to a delta state at 2:34. I remain at Level 4 for about 15 minutes until the practice ends.
We can achieve a restorative state with just 30 minutes of practice. Midday meditations of 10-30 minutes confer many benefits.
Benefit One: improved sleep
Meditation has been correlated with decreased sleep onset latency (SOL), or "sleep onset insomnia." SOL can decrease total sleep time, increasing risk for many health conditions, including heart disease, stroke, and all-cause mortality (Sharpe, 2020).
Benefit Two: restores wakefulness
A nap during the afternoon restores wakefulness and promotes performance and learning. Several investigators have shown that napping for as short as 10 minutes improves performance. Naps of less than 30 minutes confer several benefits, whereas longer naps are associated with a loss of productivity and sleep inertia (Dhand, 2006). Yoga nidra is not napping, but I suspect the benefits are the same, because the brain cycles at the same frequency.
Benefit Three: improves attention, learning, and motivation
A 2002 research paper ("Increased dopamine tone during meditation-induced change of consciousness," Kjaer et. al) found increased endogenous dopamine release in the ventral striatum during yoga nidra meditation. Dopamine is a neurotransmitter that influences learning, attention, and motivation. Increases of as much as 65% were recorded after yoga nidra. It's like a mental reset. After yoga nidra, the brain is resourced for demanding cognitive tasks.
Benefit Four: reduced anxiety
In a 2018 study, researchers took 60 college professors, between 30-55 years old, and assigned them to different experimental groups. There were significant reductions in anxiety and stress levels in those who practiced yoga nidra (Ferreira-Vorkapic, 2018).
Benefit Five: improved memory, learning, and executive functioning
Time-of-day modulations affect performance on a wide range of cognitive tasks measuring attentional capacities, executive functioning, and memory (Schmidt, et. al., 2007). In their study they found that, "in addition to reducing sleepiness, mid-day naps offer a variety of benefits: memory consolidation, preparation for subsequent learning, executive functioning enhancement, and a boost in emotional stability." I suspect the same effect would be found after a yoga nidra practice.
Benefit Six: Improved metabolic function
Our circadian clock serves as a timepiece. It synchronizes our physiology. The master clock resides in the superchiasmatic nucleus. As the sun rises, particular wavelengths of light are emitted. Shorter wavelength colors like blues and violets get scattered. This leaves colors with longer wavelengths- the yellows, oranges, and reds. Specialized, photosensitive ganglion cells in the eye prime the superchiasmatic nucleus to set the circadian clock. There are subsidiary clocks in other brain regions and peripheral clocks throughout the body. A number of processes throughout the entire gastrointestinal tract and liver appear to be under circadian control- such as nutrient uptake, processing, and detoxification (Reinke, 2016). The intestinal microbiome is also regulated by circadian rhythms, which can significantly impact immune function and metabolism (Voigt et al, 2016). Following my circadian rhythms, which are as predictable as the tides, I rest body and mind when the tide is low and I am in a trough.
Benefit Seven: Improved heart health
The heart is subject to fluctuations in demand throughout the day. Daily rhythms in cardiovascular function promote heart health. The heart delivers oxygen and nutrient rich blood to the organs and facilitates the removal of metabolic by-products (Zhang, 2020). During meditation, heart rate and metabolic activity slow. The heart functions more efficiently in a rested state. We place fewer demands on the heart when we are in a deep state of rest.
Benefit Eight: reduced stress
Glucocorticoid (GC) is an adrenal steroid hormone that controls a variety of physiological processes such as metabolism, immune response, cardiovascular activity, and brain function. GCs are released in response to stress. Even in relatively undisturbed states its circulating level varies (Chung at el, 2011). GCs have their own robust circadian rhythm, peaking in the mornings and when under stress. Practicing meditation midday, may reduce activity of pro-inflammatory markers, such as interleukin-6, C reactive protein and NF-kB (Creswell et al., 2012; Bhasin et al., 2013; Black et al., 2013; Bower et al., 2015; Rao et al., 2015; Bower and Irwin, 2016).
GC also acts as a resetting signal for the peripheral clocks, suggesting its importance in harmonizing circadian physiology and behavior (Son et al, 2011).
Even in a state of deep rest, the brain still process vast amounts of information. First, there is sensory input. Second, our mental activity, even in a seemingly task-less and idle state, the mind reminisces about past events and plans future ones. During yoga nidra, this activity is diminshed (at least experientially). Finally, there are emotions, which, even when subtle (such as feelings of serenity or uncertainty), are the products of brain activity and therefore come with an ongoing energy cost. During nidra, emotional output is also diminshed experientially.
Yoga nidra is like a computer reboot. It gives the mind a fresh start. This way of managing time, however, may not be fully embraced in certain cultures. Yet, intuitively we appreciate the rationality of the practice. If you were under a surgeon's knife, would you prefer to be operated by a physician who is rested or by one who is on hour 15 of a 16 hour shift? If you were flying in an airplane, would you prefer a rested pilot or a pilot who has been getting by on caffeine and energy drinks? This scenario is not as likely, however. Our transportation laws require pilots and truck drivers to rest. May our approach to living be as clear as the data backing this practice.