Your feet swell at night because gravity has been pulling fluid downward into your lower legs all day long. Every hour you spend sitting or standing, pressure builds inside the tiny blood vessels in your feet and ankles, pushing water out of your bloodstream and into the surrounding tissue. By evening, that accumulated fluid can make your shoes feel tight and your ankles look puffy. This is extremely common and usually harmless, but in some cases it signals something that deserves attention.
How Gravity Drives Fluid Into Your Feet
Your circulatory system constantly balances two opposing forces. Pressure inside your blood vessels pushes fluid outward through capillary walls, while proteins in your blood (mainly albumin) pull fluid back in. When you’re upright, gravity adds extra downward pressure to the blood in your leg veins. That increased pressure, called hydrostatic pressure, tips the balance so more fluid leaks out into the tissue of your feet and ankles than gets reabsorbed.
Normally, your calf muscles act as a pump. Each time you walk or flex your feet, those muscles squeeze your leg veins and push blood back up toward your heart. Sitting at a desk for hours or standing in one spot shuts that pump down. Fluid accumulates steadily, and by the time you get home and take off your shoes, you’re looking at the day’s total buildup. This type of swelling, sometimes called dependent edema, typically improves overnight once you lie flat and gravity is no longer working against you.
Venous Insufficiency: When Valves Stop Working
Inside your leg veins are one-way valves that keep blood moving upward. Over time, these valves can weaken or become damaged, allowing blood to flow backward and pool in your lower legs. This condition, chronic venous insufficiency, affects millions of adults and makes evening swelling significantly worse than what gravity alone would cause. Each year, about 1 in 50 adults with varicose veins progress to chronic venous insufficiency.
If you’ve had a deep vein thrombosis (a blood clot in a leg vein), the scar tissue left behind can permanently damage those valves, leading to the same pooling effect. Common signs beyond swelling include leg cramping at night, skin discoloration around the ankles, and a heavy or aching feeling that worsens as the day goes on. The swelling typically starts mild in the morning and peaks in the evening, which is why you notice it most at night.
Heart Failure and Kidney-Related Swelling
When the heart can’t pump blood efficiently, blood backs up in the veins, raising pressure throughout the system. The body responds by activating hormonal systems that tell the kidneys to hold onto salt and water. In the early stages this compensation helps maintain blood pressure, but over time it creates a cycle of worsening fluid overload. The result is swelling that tends to show up in both legs and feet equally, often accompanied by shortness of breath or fatigue.
Kidney disease causes a similar pattern through a different route. When the kidneys can’t filter properly, excess sodium and water stay in the bloodstream, increasing overall fluid volume. The extra fluid follows gravity to the lowest point, which during the day means your feet. Symmetrical swelling in both legs is a hallmark of these systemic conditions, as opposed to swelling in just one leg, which points toward a local problem like a blood clot or lymphatic blockage.
Medications That Cause Foot Swelling
Several widely prescribed drugs list foot and ankle swelling as a side effect. Calcium channel blockers, commonly used for high blood pressure, are one of the most frequent culprits. These medications relax the arteries leading into your capillaries but don’t have the same effect on the veins carrying blood away. That mismatch increases pressure inside the capillary beds, forcing fluid into the surrounding tissue. The swelling worsens with standing or sitting because the normal reflex that tightens blood vessels when you’re upright is also blocked by the medication.
Common anti-inflammatory painkillers (NSAIDs like ibuprofen and naproxen) promote sodium retention in the kidneys, which increases your total fluid volume and can tip the balance toward swelling. Blood pressure medications in other classes, including beta blockers and alpha blockers, can also contribute, especially at higher doses. If your feet started swelling after beginning a new medication, that connection is worth raising with your prescriber.
Pregnancy and Evening Swelling
Swollen feet in the evening are nearly universal in late pregnancy, and there are several reasons working at once. Total body water increases by 6 to 8 liters during pregnancy. Blood flow to the uterus reaches roughly 700 milliliters per minute by full term, about 16 percent of total cardiac output, which means substantially more blood is circulating through the lower half of the body.
The growing uterus also physically compresses the veins and lymphatic channels in the pelvis, making it harder for fluid to drain back up from the legs. Pregnancy is an independent risk factor for developing varicose veins and venous insufficiency. All of these factors stack on top of the normal gravity effect, which is why third-trimester swelling peaks in the evening after a day of being upright.
One Leg vs. Both Legs: What the Pattern Tells You
Where swelling appears matters. Symmetrical swelling in both feet and ankles generally points to a systemic cause: gravity-related fluid buildup, heart or kidney issues, medication side effects, or pregnancy. It’s usually not an emergency, though persistent swelling deserves investigation.
Swelling in just one leg is a different story. The two most common causes of a single swollen limb are deep vein thrombosis (DVT) and lymphedema. A DVT is a blood clot in a deep vein, typically in the calf or thigh, and it can be dangerous if the clot breaks loose and travels to the lungs. If one leg becomes suddenly swollen, warm, red, or painful, that warrants urgent medical evaluation. Chronic asymmetric swelling that develops more gradually usually suggests venous or lymphatic disease on the affected side.
How to Check the Severity
You can assess your own swelling with a simple press test. Push your index finger firmly into the skin just above your inner ankle bone and hold for about 20 seconds, then release. If the skin bounces right back, the swelling is minimal. If your finger leaves a visible dent, that’s pitting edema, and the depth of that pit corresponds to severity. A shallow indent under 2 millimeters is trace swelling. A pit of 4 to 6 millimeters is moderate. Anything deeper than 8 millimeters is considered severe and typically indicates significant fluid retention that should be evaluated.
Reducing Swelling at Home
The single most effective thing you can do is elevate your feet above heart level. Stanford Health Care recommends doing this three or four times a day for about 15 minutes each session. Lying on your back with your feet propped on pillows works well. This reverses the gravity equation, allowing fluid to drain back into your central circulation where the kidneys can process and eliminate it.
Compression socks apply gentle, graduated pressure that helps your veins push blood upward more efficiently. For everyday swelling from prolonged sitting or standing, stockings in the 15 to 20 mmHg range are effective at reducing fluid buildup. Research shows that even light compression (10 to 15 mmHg) can prevent occupational edema in people who sit or stand for long periods, though slightly higher pressures (20 to 30 mmHg) provide more significant reduction. Knee-high styles are sufficient for foot and ankle swelling. Put them on in the morning before swelling starts for the best results.
Sodium intake plays a direct role in how much fluid your body retains. The American Heart Association recommends keeping sodium under 1,500 milligrams per day for people at risk of fluid retention. For context, a single restaurant meal can easily contain 2,000 to 3,000 milligrams. Reducing processed food, canned soups, and restaurant meals can make a noticeable difference in evening swelling within days.
Moving your calf muscles throughout the day keeps the venous pump active. If you work at a desk, flexing your feet up and down for 30 seconds every hour, or taking a short walk, can meaningfully reduce how much fluid accumulates by evening. Even small, consistent movement makes a difference.