Once the days begin getting shorter, something strange can happen. You may find it harder to get out of bed in the morning, feel ready for sleep unusually early, or notice that your usual afternoon slump suddenly feels much stronger. Even if your schedule has not changed, your energy levels sometimes seem to know that winter is coming.
There is a biological reason for that. Your body uses light as one of its most important signals for deciding when to feel awake and when to prepare for sleep. As the seasons change and daylight becomes more limited, those signals change too. For some people, this can affect sleepiness, alertness, mood, and the timing of their internal body clock.
So, why can shorter days leave you feeling more tired, and is there anything you can do about it?
Your Body Runs on an Internal Clock
Your body follows an approximately 24-hour cycle known as the circadian rhythm. This internal timing system helps regulate sleep and wakefulness, but its influence extends much further. Circadian rhythms also affect hormone release, body temperature, appetite, digestion, and other biological processes throughout the day.
Your internal clock is not completely self-sufficient, though. It relies on environmental cues to stay synchronized with the outside world, and light is one of the strongest. Light entering your eyes helps signal to the brain that it is daytime, while darkness signals that nighttime is approaching. This light-dark cycle helps keep your sleep-wake rhythm aligned with the 24-hour day. (1)
During the summer, this system receives plenty of obvious cues. The sun rises earlier, evenings remain bright for longer, and many people naturally spend more time outdoors. In fall and winter, those cues become less pronounced. You may wake before sunrise, spend most of your day indoors, and return home after the sun has already set.
Morning Light Helps Tell Your Body to Wake Up
Think about the difference between waking up on a bright summer morning and waking up while it is still dark outside. In summer, sunlight may already be coming through your window before your alarm rings. In winter, you may get dressed, eat breakfast, and start your commute without encountering much natural light at all.
That difference matters because morning light helps influence the timing of your circadian rhythm. Bright light after waking can promote alertness and help reinforce the message that your biological day has begun. The CDC notes that bright light shortly after waking may help you feel more alert, while morning light can also shift the circadian system toward an earlier schedule. (1)
When mornings remain darker for longer, your usual wake-up time may therefore arrive without the strong environmental signal your body associates with daytime. Your alarm clock may say that the day has started, but your biological clock is also paying attention to what is happening outside.
This does not mean that darkness automatically makes everyone exhausted during winter. However, reduced exposure to bright morning light may be one reason some people find seasonal mornings especially difficult.
Earlier Darkness Can Change Your Sleep Signals
The other half of the light-dark cycle involves melatonin, a hormone closely connected to the timing of sleep.
As darkness increases in the evening, your brain normally begins releasing more melatonin. This helps signal that nighttime is approaching and prepares the body for sleep. Melatonin levels tend to rise during the evening and remain elevated during the night before falling again as morning approaches. The National Heart, Lung, and Blood Institute explains that darkness promotes melatonin release, while exposure to light helps regulate the sleep-wake cycle (2).
In summer, darkness may arrive relatively close to the time you begin winding down for bed. In fall and winter, however, sunset can happen hours earlier. Suddenly, you may be sitting at your desk, cooking dinner, or commuting home in conditions that look much more like nighttime than daytime.
Earlier sunsets do not mean that everyone immediately begins producing excessive amounts of melatonin. The relationship between seasonal light exposure and sleep is more complicated than that. However, changing patterns of light and darkness can affect circadian timing and may contribute to feeling sleepy earlier than usual.
You May Be Getting Much Less Daylight Than You Think
Shorter days are only part of the issue. Changes in your daily habits can reduce light exposure even further.
Outdoor daylight is substantially brighter than ordinary indoor lighting, even when the weather is cloudy. During summer, you may encounter natural light without making any special effort. You might walk after work, eat outside, exercise outdoors, run errands while it is still light, or simply spend more time near open windows and bright spaces.
That exposure can disappear surprisingly quickly once fall arrives. You may leave home before sunrise, work under indoor lighting all day, and leave after sunset. On particularly busy days, your only meaningful exposure to daylight may occur through an office window.
This matters because bright daytime light does more than help regulate your body clock. It can also support daytime alertness. Research summarized by the CDC indicates that bright light during the day can help improve alertness and may also support sleep later at night. (1)
As a result, the sleepy feeling associated with shorter days may not simply come from having more hours of darkness. It can also reflect how little truly bright light you encounter while you are awake.
Sleepiness Is Controlled by More Than One System
It is tempting to assume that feeling more tired must mean you are sleeping less, but sleepiness is not determined by sleep duration alone.
Two major biological processes work together to influence when you feel tired. The first is your need for sleep, sometimes called sleep pressure. This gradually builds during the hours that you are awake and decreases while you sleep. The second is your circadian rhythm, which influences when your body encourages wakefulness and when it becomes more prepared for sleep. The NHLBI describes these two systems as working together to determine when we feel awake and when we feel sleepy (3).
This helps explain why you can sometimes feel sleepy at an inconvenient time even if you technically slept for enough hours. Seasonal changes in light exposure can affect the timing signals coming from your circadian system without necessarily changing how long you slept the previous night.
For some people, this may show up as more difficulty waking in the morning, an earlier urge to go to bed, stronger afternoon fatigue, or a general desire to spend longer under the covers.
What About Vitamin D?
When conversations turn to winter tiredness, vitamin D often enters the discussion. There is some logic behind the association because the body produces vitamin D when the skin is exposed to ultraviolet B radiation from sunlight, and opportunities for effective sun exposure can decline during parts of the year.
However, feeling tired when the seasons change does not automatically mean that you have a vitamin D deficiency. Fatigue is a very nonspecific symptom with many possible causes, and it cannot tell you whether your vitamin D level is low.
Vitamin D may also have a more specific connection to winter-pattern seasonal affective disorder (SAD). The National Institute of Mental Health notes that people with seasonal affective disorder may have lower vitamin D levels, although the biology of the condition appears to involve several factors rather than a single nutrient (4).
If you are concerned about vitamin D deficiency, a healthcare professional can help determine whether testing or supplementation is appropriate rather than relying on tiredness alone as a clue.
Can You Feel More Awake During the Darker Months?
You cannot change the time of sunset, but you can give your body stronger signals about when daytime begins and ends.
1. Seek Out Morning Light
If possible, spend some time outside relatively soon after waking. Even a short walk can expose you to much brighter light than you would typically experience indoors. Morning light is particularly important to circadian timing and can help reinforce the transition from sleep to wakefulness.
If getting outside first thing is not practical, opening curtains and blinds as soon as you wake up is still better than spending the beginning of the day in a dark room.
2. Get Outside During the Day
Morning is not your only opportunity to benefit from natural light. If you spend most of your working day indoors, consider taking part of your lunch break outside or finding another reason to step outdoors during daylight hours.
This can become especially important during winter because opportunities for daylight exposure are limited. Waiting until the end of the workday may mean waiting until after sunset.
3. Keep Your Sleep Schedule Consistent
A regular sleep schedule helps give your circadian system predictable timing cues. Try to keep your bedtime and wake time reasonably consistent throughout the week rather than repeatedly switching between very different schedules.
Sleeping several hours later on weekends can feel appealing when mornings are dark, but large changes in sleep timing may make Monday morning even more difficult.
4. Make Evenings Look Like Evenings
Just as bright light can help signal daytime, excessive light late at night can send conflicting information. Bright room lighting and electronic screens can interfere with the normal evening rise in melatonin, particularly when exposure occurs close to bedtime. The NHLBI recommends managing artificial light exposure at night as part of maintaining a healthy sleep-wake rhythm (5).
You do not need to spend your evening sitting in darkness. Instead, consider gradually lowering the intensity of your surroundings as bedtime approaches and avoiding unnecessarily bright screens right before you plan to sleep.
5. Keep Moving
Shorter days often change activity levels along with light exposure. Cold weather, rain, and early sunsets can make an evening walk or outdoor workout less appealing, which may mean you are moving less without realizing it.
Regular daytime physical activity supports overall health and can also contribute to healthier sleep patterns. If your usual summer activities become impractical, finding an indoor alternative can help preserve some consistency in your routine.
6. Eat Healthy and Supplement As Needed
Light exposure and sleep timing are important, but your everyday nutrition can also affect how energetic you feel. Eating regular, balanced meals that include sources of protein, complex carbohydrates, healthy fats, fruits, and vegetables can help provide your body with the nutrients it needs throughout the day.
This can become more difficult during the colder months, when routines change and fresh, lighter meals may be replaced by heavier comfort foods. Rather than focusing on any single “energy-boosting” food, aim for variety and consistency. Nutrients such as iron, vitamin B12, folate, and vitamin D all play different roles in normal energy metabolism and overall health.
Supplements can help fill specific nutritional gaps, but they should not automatically be the first response to feeling tired. If your diet is lacking in certain nutrients, you have a known deficiency, or a healthcare professional has recommended supplementation, a targeted supplement may make sense. Taking more of a nutrient than you need, however, is not necessarily better.
If your fatigue is persistent or noticeably different from your usual seasonal slump, it is worth considering whether something beyond shorter days may be involved.
Don’t Blame Everything on the Season
If you start feeling tired around the same time the days become shorter, it is easy to assume that winter is responsible. Sometimes it may be part of the explanation, but persistent fatigue has many possible causes.
Insufficient sleep, stress, iron deficiency, vitamin deficiencies, thyroid conditions, certain medications, sleep disorders, and numerous other health issues can contribute to fatigue. If you are unusually exhausted, your tiredness is getting worse, or you continue feeling drained despite getting adequate sleep, it is worth discussing the issue with a healthcare professional.
Seasonal changes can influence the way we feel, but they should not become an explanation for every unexplained change in energy.