A healthy fasting blood sugar level is below 100 mg/dL (5.6 mmol/L). That single number is the most widely used benchmark, but blood sugar isn’t static. It shifts throughout the day based on what you eat, how you sleep, and how active you are. Understanding the full picture, from fasting levels to post-meal spikes to long-term averages, gives you a much clearer sense of where you stand.
Fasting Blood Sugar Ranges
Fasting blood sugar is measured after at least 8 hours without eating, typically first thing in the morning. The thresholds are straightforward:
- Normal: below 100 mg/dL (5.6 mmol/L)
- Prediabetes: 100 to 125 mg/dL (5.6 to 6.9 mmol/L)
- Diabetes: 126 mg/dL (7.0 mmol/L) or higher on two separate tests
That two-test requirement matters. A single high reading can result from stress, illness, or a poor night of sleep. A diabetes diagnosis requires confirmation.
Blood Sugar After Meals
Your blood sugar naturally rises after eating and typically peaks about 60 to 90 minutes later. In a healthy person, it rarely exceeds 140 mg/dL (7.8 mmol/L) at that peak and returns close to pre-meal levels within two to three hours. If your blood sugar is still above 140 mg/dL two hours after a meal, that’s a signal your body isn’t processing glucose efficiently.
The size of the spike depends on what you ate. A meal heavy in refined carbohydrates (white bread, sugary drinks, white rice) will push your blood sugar up faster and higher than a balanced meal with protein, fat, and fiber. This is why two people can have the same fasting number but very different post-meal patterns.
A1c: Your Long-Term Average
While fasting and post-meal readings capture a moment in time, the A1c test reflects your average blood sugar over the previous two to three months. It measures the percentage of your red blood cells that have sugar attached to them. The 2025 American Diabetes Association standards break it down like this:
- Normal: below 5.7%
- Prediabetes: 5.7% to 6.4%
- Diabetes: 6.5% or higher
An A1c of 5.7% roughly corresponds to an average blood sugar of about 117 mg/dL, while 6.5% corresponds to roughly 140 mg/dL. This test is particularly useful because it smooths out the daily highs and lows that a single finger-stick reading can’t capture. It’s also not affected by whether you ate breakfast that morning.
Why Morning Readings Can Run High
If you’ve ever tested your blood sugar first thing in the morning and been surprised by a number higher than expected, your hormones are likely to blame. Between roughly 3 a.m. and 8 a.m., your body releases a surge of cortisol and growth hormone that signals your liver to produce more glucose. This gives you the energy boost needed to wake up. It’s called the dawn phenomenon.
In a healthy person, the pancreas responds by releasing enough insulin to keep this surge in check, so fasting numbers stay under 100 mg/dL. But in people with insulin resistance or diabetes, the pancreas can’t keep up, and morning readings creep into the 100s or higher. If your morning numbers are consistently elevated while your daytime readings look fine, the dawn phenomenon is a likely explanation.
When Blood Sugar Drops Too Low
Low blood sugar (hypoglycemia) is defined as anything below 70 mg/dL. Severe hypoglycemia is below 54 mg/dL. While high blood sugar gets most of the attention, low blood sugar can be immediately dangerous, causing shakiness, confusion, dizziness, and in extreme cases, loss of consciousness.
Hypoglycemia is most common in people taking insulin or certain diabetes medications, but it can also occur in people without diabetes after prolonged fasting, intense exercise, or heavy alcohol consumption. If you feel shaky, sweaty, or suddenly confused, those are your body’s warning signals that glucose has dropped too low.
Blood Sugar Targets During Pregnancy
Pregnancy changes the equation. The thresholds for gestational diabetes are tighter than standard diabetes criteria because even modestly elevated blood sugar can affect fetal development. During a three-hour glucose tolerance test, healthy levels are:
- Fasting: 95 mg/dL (5.3 mmol/L) or lower
- One hour after glucose drink: 180 mg/dL (10.0 mmol/L) or lower
- Two hours: 155 mg/dL (8.6 mmol/L) or lower
- Three hours: 140 mg/dL (7.8 mmol/L) or lower
Notice that the fasting threshold is 95 mg/dL rather than the usual 100. Some providers use a standalone two-hour test instead, where the fasting cutoff is even stricter at 92 mg/dL. Gestational diabetes screening typically happens between weeks 24 and 28 of pregnancy, though women with risk factors may be tested earlier.
Time in Range: A Newer Way to Measure
Continuous glucose monitors (CGMs), small sensors worn on the skin that check blood sugar every few minutes, have introduced a metric called “time in range.” Instead of isolated snapshots, you see what percentage of your day falls within a target window.
For most people, the target range is 70 to 180 mg/dL. The goal is to spend at least 70% of your day within that window, which works out to roughly 17 out of 24 hours. This metric captures something that fasting readings and A1c tests miss: how much your blood sugar swings throughout the day. Two people with the same A1c can have very different daily patterns. One might be relatively stable, while the other is regularly spiking to 250 mg/dL and crashing to 60 mg/dL. Time in range reveals those differences.
Converting Between Units
If you’re reading research or glucose monitors from outside the United States, you’ll encounter mmol/L instead of mg/dL. The conversion is simple: divide mg/dL by 18 to get mmol/L, or multiply mmol/L by 18 to get mg/dL. So a fasting level of 100 mg/dL equals about 5.6 mmol/L, and a reading of 7.0 mmol/L equals about 126 mg/dL. The thresholds are the same worldwide; only the units differ.
What Affects Your Numbers Day to Day
Blood sugar is not a fixed value. It fluctuates constantly, and understanding why helps you interpret your readings without unnecessary worry. Carbohydrate-heavy meals push it up. Exercise pulls it down, sometimes for hours afterward. Stress and illness raise it because cortisol and adrenaline trigger glucose release from the liver. Poor sleep, even a single bad night, can temporarily increase insulin resistance and produce a higher fasting reading the next morning.
Dehydration concentrates glucose in your blood, which can make readings appear higher. Caffeine affects some people’s blood sugar more than others. Even the order in which you eat your food matters: eating vegetables and protein before carbohydrates in the same meal produces a noticeably smaller glucose spike than eating the carbohydrates first. A single out-of-range reading in the context of otherwise healthy numbers is rarely meaningful. Patterns over weeks and months tell the real story.