CAN A NIGHT-CAP CONTAINING CARBOHYDRATES IMPROVE SLEEP?

I first became aware of the connection between carbohydrates and sleep when a dietitian colleague at the University of Sydney, Dr Helen O’Connor, published a paper in the American Journal of Clinical Nutrition in 2007 (1). I have always had trouble falling asleep whenever I am stressed. It started with exams as a teenager and got worse during my PhD. If the composition of the dinner meal was important, I wanted to know about it.

I’m not alone. According to one Gallup poll, at least 1 in 2 people do not sleep well at least once or twice a week and one in 10 have long term sleep difficulties. I had tried everything from counting sheep (even as a 6-year-old) to supplements (melatonin, St John’s wort) to prescription drugs (used cautiously). Foot massages from a generous husband usually worked but were not the best for his sleep. I learned to avoid caffeine after lunchtime. Later, I did an 8-week course in mindfulness that I still practice routinely. Nowadays, cognitive behavioural therapy is considered best practice for treating insomnia (2). But surely some foods would help too?

Food has long been thought to influence sleep. My mother used to tell me to drink warm milk, often with a malted powder such as Horlicks (3). The theory was that proteins containing the amino acid tryptophan, which is a precursor of brain serotonin (the calming hormone), would be sleep-inducing. It was also known that the ratio of tryptophan to certain other amino acids promoted the entry of tryptophan into the brain.

This is where carbohydrates come into the story. Hyperglycaemia or high blood glucose (e.g. a spike in blood glucose levels) helps to increase the ratio of circulating tryptophan in the blood through the downstream actions of insulin. Apart from promoting glucose uptake, insulin has a primary role in pushing certain amino acid into muscle, excluding tryptophan. Thus, a meal based on high GI carbohydrates (such as most potatoes) would be expected to tip the ratio in favour of higher tryptophan and therefore serotonin in the brain, helping you to nod off to sleep.

Helen and her colleagues, Chin Moi Chow and Afaghi Ahmad, compared sleep quality in 12 healthy young adults after meals based on low GI or high GI carbohydrates. Both meals were consumed 4 hours before usual bedtime. As suspected, they found that ‘time-to-fall-asleep’ (sleep latency) was faster by about 8 mins after the high GI meal compared to the low GI meal. It was also faster if the meal was consumed 1 hour before usual bedtime (but that’s not such a good idea for other reasons, like reflux). There were no other differences in sleep quality as judged by the total sleep time, total wake time or the proportion of time in difference sleep stages or dreaming (REM vs non-REM sleep).

Sleep quality is critical for mental health, physical performance and overall well-being (4). Studies show that we cycle through the various sleep stages approximately every one-and-a-half hours, with most people getting 5 cycles or about 7-8 hours altogether. If we have too little Stage 4 sleep (deep sleep) or too much REM sleep, cognition and mood are impaired. Parents with new babies are well acquainted with this phenomenon!

Interestingly, sleep quality also influences feelings of hunger (5) and blood glucose levels, increasing glucose intolerance (6). Even in our GI testing lab in Sydney, with lean young healthy participants, we were able to detect the difference in glucose tolerance between those who were asked to cut sleep to 5 hours vs usual sleep time. Sleep deprivation was associated with a higher glucose area under the curve, indicating that it took longer to restore baseline glucose levels, likely because of increased insulin resistance.

Thus, sleep has the ability to affect our weight through stimulating higher food intake and lowering insulin sensitivity. In our current environment of food abundance and sedentary lifestyles, adding sleep deprivation to the mix is a recipe for trouble.

This brings me to the most recent paper comparing high and low GI meals on sleep quality. This time the researchers used actual tests of cognition to determine if there were effects of the glycemic index on sleep and mental performance (7). The study was led by Chantelle Gaylor and colleagues at Swansea, Nottingham Trent and Edinburgh Universities in the UK. Instead of meals, they actually fed drinks containing 50 g of a low GI sugar (Palatinose, GI = 32) versus a high GI sugar glucose (GI = 100). Twenty healthy males attended the sleep lab on 3 occasions, one week apart. The first was simply to acclimatise the subjects to the lab. The second and third nights were randomised to high or low GI drinks.

First up, there appeared to be no differences between the drinks, including no effect on time-to-fall-asleep. But when they looked more closely, they noted that the order in which the participants received the drinks (low GI first or high GI first) seemed to have a significant bearing on the results. The proportion of sleep in Stage 3 was significantly longer (29% vs 24%) after the low GI drink than high GI drink, but only when the low GI drink was consumed second. It’s possible by the third night, participants were more acclimatised to the metabolic sleep ward, allowing the effects of glycemia to be detected. More tellingly, overnight recall of the details of a story was better when the low GI meal was consumed on the 3rd night. In my opinion, these results are telling us something important but, as is often the case in science, we need further studies.

A final note:

This story is dedicated to my colleague Helen O’Connor who died far too young in 2020 from ovarian cancer. She was not only a lovely person, she was a sports dietitian who coached winning Australian footballers and ballet dancers and co-authored one of our books on low GI diets in sport. Research on ovarian cancer is still grossly under-funded (8). Consider a donation on Mother’s Day this year.

Read more:

1. Afaghi and colleagues. High-glycemic-index carbohydrate meals shorten sleep onset. The American Journal of Clinical Nutrition, 2007.
2. Mitchell and colleagues. The impact of cognitive behavioural therapy for insomnia on objective sleep parameters: A meta-analysis and systematic review. Sleep Medicine Reviews, 2019.
3. Horlicks
4. Palmer and colleagues. Sleep loss and emotion: A systematic review and meta-analysis of over 50 years of experimental research. Psychol Bull. 2024.
5. Schmid and colleagues. A single night of sleep deprivation increases ghrelin levels and feelings of hunger in normal-weight healthy men. J Sleep Res. 2008.
6. Ibasaraboh and colleagues. Associations between poor sleep and glucose intolerance in prediabetes. Psychoneuroendocrinology, 2019.
7. Gaylor and colleagues. Low and high glycemic index drinks differentially affect sleep polysomnography and memory consolidation: A randomized controlled trial. Nutr Res. 2025.
8. Ovarian cancer.

Emeritus professor Jennie Brand-Miller held a Personal Chair in Human Nutrition in the Charles Perkins Centre and the School of Life and Environmental Sciences, at the University of Sydney until she retired in December 2022. She is recognised around the world for her work on carbohydrates and the glycemic index (or GI) of foods, with over 300 scientific publications. Her books about the glycemic index have been bestsellers and made the GI a household word.