Sleep is so critical for day to day physiological functioning. Without sleep, our immune system is dampened, our metabolic processes slowed and our mental health and cognition is drastically compromised. (1,2) Sleep deprivation also correlates with a heightened risk of developing certain diseases including obesity, type 2 diabetes, mood disorders (anxiety and depression) and cardiovascular diseases like hypertension. (2)
So how much sleep do we actually need? The amount of sleep we need varies depending on age but research suggests an average of 7-9 hours of good quality sleep per night is best. (3)
Sleep is influenced by our natural sleep-wake cycle, one of the body’s circadian rhythms. (4) The sleep-wake cycle is under hormonal control, notably being influenced by our sleep hormone melatonin, secreted by the pineal gland. Melatonin peaks nocturnally and diminishes during the day when there is more daylight. Therefore, environmental daylight is a pivotal regulator of our sleep-wake cycle. (4)
Melatonin secretion is also dependant on the availability of its precursor, tryptophan which is an essential amino acid that must be consumed in the diet. (4)
Nutrition and Sleep Quality
Diet and sleep are inherently linked. It has been demonstrated that even mild sleep-deprivation can lead to increased day-time food intake and weight gain. (5,6)
So how does what we eat impact sleep?
Tryptophan Rich Foods (and the importance of Vitamin B6)
As tryptophan is a precursor for melatonin, research has analysed the effects of tryptophan rich diets on sleep quality. Tryptophan can cross the blood brain barrier and is firstly converted to 5-hydroxytryptamine (serotonin) prior to being converted to melatonin in the pineal gland. (7) This process can be facilitated by vitamin B6 which acts as a co-enzyme in this tryptophan-serotonin pathway. (8)
One study analysed the effects of having a: tryptophan-rich breakfast in combination with either a dim-light day-time environment (Rich*Dim group) or a brightly-lit day-time environment (Rich*Bright) or a low tryptophan breakfast with either a dim-light day-time environment (Poor*Dim) or a brightly-lit day-time environment (Poor*Bright). (7) The tryptophan rich breakfasts utilised food protein sources including salmon and natto (traditional Japanese fermented soybeans). After four days, saliva melatonin levels were significantly less than on the first day in the Poor*Dim group, whereas melatonin was significantly higher in the Rich*Bright group. This demonstrates that a tryptophan-rich breakfast along with bright-light exposure during the day enhances melatonin secretion. (7)
A Japanese study involving 63 subjects found that intervening for one month with a protein-rich breakfast (including foods like fermented soybeans) and vitamin B6-rich
foods (such as bananas) at breakfast; along with sunlight exposure after breakfast and
incandescent light (low-temperature light) exposure at night significantly improved salivary melatonin levels resulting in easier sleep onset at night and higher quality of sleep. (9)
A study which was similar to the above, analysed the effects of a high tryptophan rich and vitamin B6 rich breakfast followed by morning sunlight exposure on sleep patterns, but this time on children aged 2-6 years. (8) Results supported the notion that tryptophan and Vitamin B6 intake along with morning light exposure may enhance the synthesis of serotonin and have a natural sleep-inducing effect when converted to melatonin in the evening. It is theorised that morning sunlight exposure accelerates the synthesis of serotonin from tryptophan in the pineal gland. (8)
· Food sources of tryptophan are high protein foods including eggs, milk, tofu and traditional fermented soy products, salmon and other fish, turkey, other meat, beans and wholegrains.
· Food sources of Vitamin B6 include fish, meat, bananas and nuts and seeds.
Iron
One mineral deficiency that has demonstrated correlations with poor sleep is iron; particularly in children. (10)
A study on children with attention-deficit/hyperactivity disorder (ADHD) who often have sleep disturbances analysed serum ferritin levels (iron stores). It was observed that of 68 children, those with serum ferritin levels <45 µg/l had significantly higher “sleep wake transition disorders” scores, which encompasses abnormal movements in sleep. (10)
Interestingly, another paper compared sleep spindle patterns occurring in non-rapid eye-movement (NREM) sleep stages in 6-month-old iron-deficient anaemic and non-anaemic infants. (11) Sleep spindles have been investigated as being a marker of normal neurodevelopment, and their absence or abnormality strongly implies cerebral dysfunction. It was found that iron deficient infants had sleep spindles with reduced density, lower frequency, and longer inter-spindle intervals in NREM sleep stage 2 and slow-wave-sleep. (11)
In a case control study, parent-reported sleep issues in 41 children with ADHD aged 6 to 12 years were compared to those of 62 children without ADHD; serum ferritin levels were analysed against sleep symptoms in both groups. (12). Children with serum ferritin levels <30 ng/mL had more frequently disturbed sleep. Significant negative correlations between sleep duration and serum ferritin levels were observed. (12)
Macronutrient Make-up
Protein
As tryptophan is an amino acid derived from protein, variations in macro-nutrient intake may affect sleep. Tryptophan is transported across the blood brain barrier by transporters that are also utilised by large neutral amino acids (LNAA) so they compete for absorption, therefore, the ratio of tryptophan to LNAA will determine the amount of tryptophan able to be taken up into the brain. (13) A high protein meal, containing more tryptophan than LNAA may assist melatonin levels.
Carbohydrate
Carbohydrate ingestion may increase tryptophan and thus melatonin. Carbohydrate ingestion triggers insulin secretion to stimulate LNAA transportation into skeletal muscle tissue for use, leaving free tryptophan to cross the blood brain barrier. (13)
Other studies have supported the notion that a high carbohydrate meal supports sleep quality. One study analysed the result of providing a group of men a high glycaemic index (GI) meal compared to a low GI meal either 4 hrs or 1 hr prior to sleep. (14) The high GI meal eaten 4 hours before sleep significantly improved sleep-onset latency compared to the low GI meal. Providing this meal 4 hours before sleep demonstrated better results than 1 hour prior to sleep. (14)
Finally, in one study 44 participants were designated diets (either high-fat, high-carbohydrate, high-protein or control) over four days and their sleep was monitored. (15) It was found that those consuming a high carbohydrate diet had significantly reduced sleep onset latencies than controls and that high-protein diets were greatly associated with significantly fewer waking episodes in the night. (15)
Therefore, macronutrient intake at dinner may affect sleep and based off the above research, a high-protein (specifically tryptophan) meal or a high-carbohydrate meal consumed at least an hour prior to sleep may benefit sleep quality.
Note: High carbohydrates doesn’t have to mean white bread and white pasta! Incorporate wholefood, un-refined carbohydrates sources such as sweet potato and potatoes, brown rice, beetroots, quinoa, fruits and other vegetables.
Zinc
Zinc is another mineral that has been investigated in relation to improving sleep quality. Some studies have demonstrated a correlation between zinc deficiency and poor sleep-quality.
In 126 adult Korean women, zinc and copper levels in the serum and hair were measured and sleep quality was analysed. (16) Interestingly, the women with the highest serum and hair zinc to copper ratio had the greatest percentage of optimal sleep amounts (7–7.9 h). (16)
A cohort study on 1295 Chinese children examined blood zinc levels and subjective sleep data, collected when the children were at preschool age (3-5 years old) and early adolescence (11-15 years old). (17) Blood zinc levels correlated with sleep duration and sleep quality in early-adolescent age but not at pre-school age. It was also found that zinc levels at pre-school age were predictive of sleep quality later in life at pre-school age. (17)
A study demonstrated the effects of zinc supplementation for one month on sleep quality in 54 intensive care unit nurses. (18) Subjective sleep latency and sleep quality scores were significantly reduced in the intervention group demonstrating that zinc supplementation greatly improves sleep quality. (18)
A recent study evaluated intervention with zinc rich foods (oysters and zinc-containing yeast extracts) in 120 Japanese subjects. (19) After three months, it was found that supplementation daily with these foods improves sleep onset latency and sleep efficiency when compared to the control group. (19)
Magnesium
Magnesium is also commonly administered to assist with sleep. A study on long-term care residents found that in 43 participants with primary insomnia, those who took a prescribed supplement containing melatonin (5mg), magnesium (225 mg), and zinc (11.25 mg) for eight weeks, had remarkably better sleep. (20) Significant improvements were seen in getting to sleep, sleep quality, waking up more refreshed, alertness the following morning and total sleep time. (20)
The reason for the above results may be that zinc and magnesium enhance serotonin conversion to melatonin by binding to, and activating the enzyme needed for this conversion to take place- arylalkylamine N-acetyltransferase (AANAT). (21)
Natural Melatonin Containing Foods
Lastly, some foods do naturally contain melatonin, especially plants. These melatonin-containing foods include tomatoes, barley, olive oil, strawberries, rice and walnuts. (22) Cow’s milk is also a natural source of melatonin; hence a traditional night-time beverage is warmed milk. It has been demonstrated that “night-time milk” obtained from milking cows at night-time contains significantly greater amounts of melatonin and tryptophan! (22)
Written by Annabel Murray as part of her 2020 internship with Wholefood Healing.
References
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