You’re tired throughout the entire day. Energy stores are low, creativity is nowhere to be found, and concentration? MIA.

This universal feeling is a telltale sign of poor sleep, and we’ve all been there. But what if it feels, by all accounts, like you DID get a good night’s rest with sufficient hours of sleep? Why the lack of energy?

It's important to first differentiate between "tired" and "sleepy." According to Proper's Head Sleep Science Advisor, Allison Siebern, PhD, CBSM, the former entails low energy that makes it difficult to get moving, whereas the latter makes one feel like dozing off.

With something as complex as sleep, there’s rarely a simple, singular solution. Lifestyle plays an important role, as do certain external factors, medical conditions, and hormonal fluctuations. But these seven common reasons are a good place to start.

By “hygiene,” we mean the behavioral actions occurring during the day, specifically in the hours leading up to bedtime, that could negatively impact quality of sleep. This is something that Whitney Stuart, MCN, RDN, CDE often talks to her clients about when they come to her complaining of afternoon slumps.

“A lot of my patients lean on the fact that they take melatonin, for example. But they’re not assessing the whole picture. So we look at sleep hygiene to see if it actually was a good night’s sleep. Were they in a cold, dark room? Are they on their screens before bedtime? A lot of my patients also use either an Oura Ring or a WHOOP to assess REM sleep and see how long of the time they were in bed was actually good, restful sleep. By nature, we just want to add a quick fix, but I find that it’s probably more about the allowance of time and the prioritization of the space for sleep that matters more. No supplement will work unless we teach the action that goes with it—the how and not just the what.”

Actionable tips for lifestyle change:

  • Turn the clock around before you go to sleep and avoid the temptation to look at the time should you wake up in the middle of the night.
  • Dim bright light exposure 1-2 hours prior to bedtime. This way, the bright light won’t confuse the brain signal that regulates our circadian rhythms.
  • Use the bed for sleep and sex only, not as a hangout area. If this is impossible due to limited space or mobility, have a daytime look for the bed that you swap out at night (e.g., different pillows, sitting up rather than laying down, etc).
  • Get on a consistent sleep schedule by going to bed and getting up at the same time every day—even on weekends. This avoids confusing the body and throwing off physiological processes that thrive off the routine of a 24-hour day.

READ MORE: A Guide To The 4 Stages Of Sleep (REM + NREM)

Pair behavioral change with smart supplementation

To improve sleep quality AND quantity.

If you’ve ever taken a traditional, over-the-counter sleep aid with antihistamines, you may be familiar with that knocked-out feeling. And while it may lead some to believe they’re sleeping more (and better), science says otherwise. In fact, a Consumer Reports study (1) found that most OTC sleep aids only increase total sleep duration by a marginal 20-30 minutes—which isn’t great, but it isn’t horrible either. And yet, when you dig deeper, you’ll find that sleep time is only half the story. Quality of sleep matters, too. And when OTC sleep aids with antihistamines are involved, quality suffers.

Get deeper, better quality, restorative sleep

That leaves you feeling refreshed and well-rested.

In order to understand why, we have to talk about acetylcholine, a neurotransmitter that plays a critical role in rapid eye movement sleep, otherwise known as REM sleep. (2) During this time of the night, brain waves are far more active, although muscles remain fully relaxed—which is a good thing! It’s prime time for dreams to occur, and we don’t want to be acting those out. Because of the disparity between the brain's increasing activity and the muscles remaining inactive (known as atonia), REM is sometimes referred to as “paradoxical sleep.” We spend about 20-25% of our time in this stage, which benefits cognitive functions such as memory consolidation, creativity, and learning. (3-4)

Because antihistamines block acetylcholine, (5) REM sleep suffers. Instead, what you’re getting is more non-REM sleep, which is one reason why upwards of 40% of people taking OTC sleep aids report feelings of next-day fogginess or drowsiness. (6)

READ MORE: How Safe + Effective Are Most Over-The-Counter Sleep Aids?

Over-the-counter sleep aids affect sleep quality.

Sleep apnea is marked by temporary pauses in breathing throughout the night. Unlike the less common Central Sleep Apnea, which occurs when the brain fails to send correct signals to the muscles in charge of breathing, Obstructive Sleep Apnea (OSA) causes throat muscles to relax, partially block the airway, and lead to snoring. It’s particularly common among menopausal women—studies indicate that about 20% develop some form of the condition. (7)

What makes OSA so tricky is that many people don’t know they have it—for some, the only symptom is unexplained fatigue, while others may experience sleep disruptions due to snoring or a feeling of gasping for air.

Suspect this may be affecting you? Consult your healthcare provider or sleep medicine specialist, who may suggest that you enroll in a sleep study (aka polysomnography).

According to Dr. Siebern, this entails a lab overnight stay in which several EEG sensors are placed on the head to identify brainwaves and differentiate when someone is awake or asleep and what stages of sleep they are in. There are other sensors that also track breathing, snoring, limb movement, and heart rate across the night. Although most commonly used to diagnose sleep apnea, an in-lab polysomnography can also be helpful in the assessment and diagnosis of narcolepsy (more on that below).

It's also possible to conduct a home sleep study, which involves fewer sensors and does not require an EEG to measure brainwave activity but only looks at respiratory effort across the night and movement to estimate if someone is awake or asleep.

READ MORE: When To See A Sleep Specialist: 6 Signs + Symptoms

As the name suggests, chronic fatigue syndrome (CFS) manifests as extreme tiredness that 1) doesn't improve with rest and 2) cannot be traced back to an underlying medical condition—making it all the more difficult to diagnose. Emerging evidence points to stress, hormonal imbalances, viruses, and weak immunity as potential causes of CFS; however, your doctor will likely first screen for a myriad of other medical and mental health conditions that can contribute (see below).

Low energy or fatigue can have many sources outside of sleep disruption, explains Dr. Siebern.

"Think of the human system as similar to a battery. Sleep is one way to recharge the system; however, there can be conditions that quickly drain the battery, leading to someone having less energy and resources to manage the daytime and/or needing to rest periodically to recharge the system."

Allison Siebern, PhD, CBSM

One of the main side effects of Hashimoto's thyroiditis (aka hypothyroidism, or underactive thyroid), for example, is tiredness—as well as weight gain, joint pain, muscle weakness, constipation, dry skin, and sensitivity to cold. Your doctor can screen for thyroid problems via a simple finger-stick blood test, which measures thyroid-stimulating hormone (TSH) levels.

Other medical and mental health conditions that can lead to fatigue include Lupus, depression, anxiety, and elevated stress levels.

Even if you're getting enough sleep, energy levels may be low due to a lack of iron and/or vitamin D—both of which can be assessed via a standard blood test.

  • Iron is a mineral that helps transport oxygen to cells, tissues, and organs. Significant iron deficiency may result in anemia (also spelled anaemia), a condition in which blood lacks an adequate supply of red blood cells.
  • Low levels of vitamin D (the sunshine vitamin) may also cause feelings of daytime sleepiness since this vitamin supports the powerhouse of the cell, known as the mitochondria.

Interested in just how powerful a balanced diet can be on the quality and quantity of your shut-eye? Read up on the 15 best and worst foods for good sleep.

Low levels of vitamin D (the sunshine vitamin) may also cause feelings of daytime sleepiness.

If you feel somewhat rested upon waking, only to become increasingly sleepy throughout the day—so much so that you find yourself falling asleep while doing routine tasks—you may have narcolepsy. This chronic neurological condition impacts the brain’s ability to regulate the sleep cycle (also known as the circadian rhythm) and, as a result, causes interrupted sleep throughout the night. Other symptoms include sudden muscle weakness (cataplexy), vivid hallucinatory dreams, and paralysis just before or after sleep.

Because narcolepsy has several potential causes (e.g., family history, past brain injury, autoimmune disorders), sleep doctors typically conduct a full clinical examination and medical history assessment, which also help rule out or identify other contributing neurological conditions. Doctors may also recommend a two-week sleep journal in order to confirm cataplexy, which is unique to narcolepsy.

Available diagnostic tests include:

  • Polysomnograph (PSG): An overnight sleep study to record brain activity, breathing patterns, eye movements, and muscle contractions. This helps screen for conditions such as sleep apnea, narcolepsy, and circadian rhythm disorders.
  • Multiple sleep latency test (MSLT): A test of daytime sleepiness that measures how quickly you fall asleep and whether you enter into REM sleep.