For this study, researchers used a mouse model affixed with minimally invasive microchipsTrusted Source and a video recording system to record the physiological and behavioral habits of the mice over an extended period of time.
In their natural state, scientists observed the mice consistently have a “siestaTrusted Source” or short nap during the latter part of their awake phase. When caffeine was administered, the mice no longer took the short nap.
“These mice — and possibly some humans — are genetically programmed to have a ‘siesta’ as part of their normal sleep pattern,” Dr. Andrew Charles, professor of neurology at the University of California Los Angeles and one of the senior authors of this study explained to Medical News Today.
“You also found mice compensated for the delayed sleep onset caused by daily caffeine by sleeping more solidly and ‘sleeping in’,” he said.
Researchers also found daily caffeine administration shifted the onset of sleep in the mice — particularly REM sleep — by up to two hours relative to the light-dark cycleTrusted Source.
During the study, scientists also discovered the brain blood flow of the mice was higher when they were awake and lower during sleep. However, there was an exception during periods of REM sleep when there were large increases in brain blood flow.
“We found that under control conditions (i.e. no caffeine) REM sleep was associated with large increases in brain blood flow, which we think may be involved in the function of REM sleep,” Dr. Charles explained.
Mice that had consumed caffeine actually had a reduction in brain blood flow while they were awake and a significant boost in brain blood flow during sleep.
“Caffeine consumed during the awake state in mice resulted in a significant increase in brain blood flow during non-REM sleepTrusted Source, and augmented the increase in blood flow during REM sleep,” he continued. “We speculate that this increased brain blood flow could play a role in neuroprotectiveTrusted Source effects of caffeine by enhancing the clearance of brain wasteTrusted Source during sleep.”
To consume caffeine, most people drink beverages that include the stimulant, including coffee, tea, energy drinks, hot chocolate, and soft drinks. There are also some protein bars and even some medicationsTrusted Source that contain caffeine.
The U.S. Food and Drug Administration (FDA) suggests adults only consume up to 400 milligramsTrusted Source of caffeine a day, which is the equivalent of about four to five cups of coffee.
When caffeine enters your system, it starts to take effect rather quickly, reaching its peak in the blood within 30 to 60 minutesTrusted Source. The effects of caffeine can include increased heart rateTrusted Source, breathingTrusted Source, physical energyTrusted Source, and mental alertnessTrusted Source. How long these effects last may depend on the person and how much caffeine was taken. Caffeine typically has a half-life of five hours, meaning it takes that amount of time for the body to eliminate half of it.
If a person consumes too much caffeine, they may experience some negative effectsTrusted Source, including:
- fast heartbeat
- upset stomach
Past studies show moderate caffeine consumption can have a positive effect on weight lossTrusted Source and sports performanceTrusted Source, and may help protect the body against diseases such as Alzheimer’s diseaseTrusted Source, liver diseaseTrusted Source, cataractsTrusted Source, kidney stonesTrusted Source, and even certain types of cancerTrusted Source.
Other research has shown caffeine may cause some potentially harmful effects, such as depression and anxietyTrusted Source, increased blood sugar levelsTrusted Source, migraine headachesTrusted Source, and a negative impact on pregnancyTrusted Source.
Previous studies show caffeine can also have a negative impact on a person’s sleep. A study published in June 2023 found caffeine consumption reduces a person’s total sleep time by 45 minutes and sleep efficiency by 7%.
ResearchTrusted Source published in May 2021 reports regular daytime caffeine intake affects REM sleepTrusted Source regulation in men, delaying the start of circadian REM sleep, and also worsens the quality of awakening.
As mice have different sleep-awake cycles — and most of us are not able to “sleep in” on weekdays — how might these findings translate into humans? Should we stop consuming caffeine after 2 p.m.?
“Many individuals perceive that caffeine, particularly if consumed later in the day, interferes with their sleep quality,” Dr. Charles answered. “Our studies show that caffeine delays the onset of sleep, and particularly REM sleep, but once the mice fell asleep, they, in fact, slept more solidly, and they compensated for the delayed onset of sleep by sleeping later in their ‘morning’.”
“For the majority of us who are not able to sleep later on most days, this delayed onset of sleep [due to caffeine consumption] could be disruptive, which may explain why for some individuals, limiting caffeine to the morning hours is important to avoid sleep onset.”
— Dr. Charles
Dr. Monique May, a board certified family physician and medical advisor for Aeroflow Sleep, who was not involved in this study, told MNT it is hard to give definitive suggestions for humans based on this data as the study size was small and the 24-hour day-night cycleTrusted Source — with 12 hours lights on and 12 hours lights off — is not likely to be typical for most adults in today’s society.
“In general, the recommendation is to cut off caffeine four to six hours before your bedtime. However, that number may need to be increased based on factors such as age, one’s metabolism, underlying medical conditions, how much caffeine one drinks, and whether one smokes cigarettes.”
— Dr. Monique May
And because the effects of caffeine can last for a few hours, Dr. Michael Gallo, a sleep medicine physician at Baptist Health South Florida, suggested only consuming caffeine in the morning and at least 12 hours before your bedtime.
“It is possible caffeine can delay your sleep onset and disrupt your sleep architecture if you drink coffee or consume anything caffeinated within 12 hours of bedtime. Caffeine competitively competes and inhibits the effects of a neurotransmitter called adenosineTrusted Source; that is, caffeine binds to adenosine’s receptors and prevents adenosine from acting on these receptors,” he explained.
“Adenosine is the molecule that makes us feel sleepy, so if this molecule cannot function by binding to its receptors, then we will not feel sleepy, hence the goal of caffeine. Therefore, if caffeine is still in your bloodstream prior to bedtime, you will likely not achieve consolidated and deep sleepTrusted Source, and could delay your sleep onset,” Dr. Gallo added.