May 5, 2026, 2:59 pm | Read time: 5 minutes
Supplements are a major topic among athletes and health-conscious individuals. Many take them daily, often at the same time each day. However, there are numerous recommendations about the best time to take them: Some suggest certain supplements should be taken in the evening, while others are better in the morning. FITBOOK examined four supplements that are often recommended not to be taken in the evening and reviewed the research on them.
Timing of Supplement Intake Widely Discussed, but Little Researched
Dietary supplements are intended to complement the diet with vitamins, minerals, amino acids, or plant extracts. For many, these small capsules and tablets have become a regular part of daily life. There are numerous recommendations, particularly advising against taking vitamin D, B vitamins, and activating plant extracts right before bedtime.
The problem is that many of these claims sound plausible but have been scarcely studied scientifically. While there are often studies on the dosage and effects of individual substances, there is significantly less research on whether the timing of intake also plays a role. So, what holds up scientifically–and what is more assumption than evidence?
Clear Situation with Caffeine in the Evening
With caffeine, the data is clear. It blocks adenosine receptors in the brain, reducing “sleep pressure.” Adenosine is a neurotransmitter that increases throughout the day and signals fatigue.1
When its effect is inhibited, the natural sleep pressure is delayed. A 2013 study on twelve healthy adults examined how taking 400 milligrams of caffeine 0, 3, or 6 hours before bedtime compared to a placebo affects sleep under everyday conditions. Caffeine significantly shortened total sleep time at all three times–even when taken six hours before going to bed. This supports the recommendation to avoid caffeine at least six hours before bedtime.2 A meta-analysis from the University of Zurich also confirms that caffeine prolongs the time it takes to fall asleep, worsens sleep quality, and reduces deep sleep.3
There is clear evidence that caffeine-containing supplements should be avoided in the evening.
Vitamin D is Theoretically Problematic, Empirically Unproven
A common recommendation is to take vitamin D in the morning, as it might disrupt sleep. This is based on vitamin D’s effect in the brain through its receptor (VDR), including in the suprachiasmatic nucleus, the central “pacemaker” of the circadian rhythm. There, vitamin D could influence so-called clock genes that regulate the sleep-wake cycle. Additionally, vitamin D affects enzymes involved in melatonin synthesis, leading to theoretical discussions that it could interfere with the internal clock and sleep regulation.4
Indeed, vitamin D receptors exist in the brain, which could theoretically be involved in sleep regulation. However, clinical studies do not provide a clear picture.
A study from the University of Tromsø found no evidence that vitamin D supplementation significantly affects sleep. However, the timing of intake was not examined, so no recommendation can be made for morning or evening.5
B Vitamins are “Activating,” but Not Clearly Sleep-Disrupting
B vitamins are involved in energy metabolism, which is why it is often recommended to take them in the morning. However, there is no convincing clinical evidence that taking vitamin B12 in the evening disrupts sleep. Randomized studies with clearly defined sleep-related endpoints are largely lacking, and the available data do not consistently indicate a sleep-disrupting effect.
A study from the University of Crete examined the relationship between serum vitamin B12 levels and sleep parameters, but not the timing of intake. Therefore, it does not provide any insight into whether morning or evening intake makes a difference. Instead, it only shows that lower vitamin B12 levels were associated with insomnia symptoms–especially in older individuals, women, and non-obese people–as well as with daytime sleepiness in obese individuals, but not with overall poorer sleep quality.6
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Adaptogens–Between Stimulant and Sleep Aid
Ginseng and Rhodiola
Adaptogens are natural, mostly plant-based substances like herbs, roots, and mushrooms that are supposed to help the body adapt to physical and mental stress. Plant-based adaptogens like ginseng or rhodiola are considered performance-enhancing or fatigue-reducing. Therefore, it is often assumed that taking them in the evening could disrupt sleep.
A Swedish animal study suggests that some adaptogens, including rhodiola, schisandra, and eleutherococcus, can indeed increase alertness or reduce fatigue.7
Ashwagandha
However, adaptogens are not inherently synonymous with “stimulating.” Ashwagandha is also considered an adaptogen but is more associated with stress-reducing and calming effects. Clinical studies have shown ashwagandha improves stress, cortisol levels, and, in some cases, sleep parameters.
In an Indian study with 60 patients with insomnia, taking 300 milligrams of ashwagandha extract twice daily for ten weeks led to a significant reduction in time to fall asleep, improved sleep efficiency, and better subjective sleep quality compared to a placebo. However, the specific impact of exclusively evening intake was not examined.8
The effect of adaptogens on sleep is thus partly activating, partly calming. There is currently no sufficient scientific evidence regarding the optimal timing of intake.
Conclusion: More Myth than Must
The scientific assessment is nuanced: Caffeine and other strongly stimulating substances are clearly considered sleep-disrupting. For vitamin D, B vitamins, and adaptogens, there are mainly theoretical considerations, but no convincing evidence that evening intake actually impairs sleep.
For many supplements, there is simply a lack of high-quality studies on the question of “morning or evening?” Those who want to act based on evidence should be particularly cautious with clearly stimulating substances. Most other timing recommendations sound plausible but have not yet been scientifically confirmed or clearly refuted.