A – Z
These are the terms you will encounter across DayByDay — in the app, in your data, and in this help center. Definitions are kept short and jargon-free on purpose.
A1c (HbA1c)A blood test that shows your average blood sugar level over the past 2–3 months. Reported as a percentage. Lower is generally better for people managing blood sugar.Active caloriesCalories burned through movement and exercise, on top of what your body burns at rest. Synced from Apple Health.AHI (Apnea-Hypopnea Index)The number of times per hour you stop breathing or have significantly reduced breathing during sleep. AHI is the main metric for diagnosing sleep apnea. DayByDay does not currently track AHI directly — it focuses on blood oxygen and sleep stages instead.Apple HealthApple's built-in health data platform on iPhone and Apple Watch. DayByDay can read weight, steps, sleep, workouts, and heart rate from Apple Health automatically.BMI (Body Mass Index)A number calculated from your weight and height. It is a rough screening tool — not a precise measure of health. Muscle, age, and ethnicity can all affect whether BMI accurately reflects your body composition.BMR (Basal Metabolic Rate)The number of calories your body burns just to stay alive at complete rest. Think of it as your body's idle speed. DayByDay calculates this using the Mifflin–St Jeor equation.Body fat percentageThe proportion of your total body weight that is fat tissue. You can log this manually (from a smart scale or DEXA scan) or let DayByDay estimate it using your waist and height measurements.Coach NotesPersonalized, plain-English insights generated from your real data — things like "You're averaging 2.1 lbs/week" or "Your sleep improved after your last dose increase." These update automatically as your data changes.Core sleepLight sleep stages (N1 and N2) that make up the bulk of a typical night. Core sleep is important for memory consolidation and general rest, though deep sleep and REM are considered higher quality.Deep sleepThe most physically restorative stage of sleep (N3, also called slow-wave sleep). Your body repairs tissue, builds bone, and strengthens the immune system during deep sleep.Drops per hour (oxygen dips)The number of times per hour your blood oxygen level drops by 3% or more during sleep. A high drops-per-hour count can be a sign of sleep-disordered breathing.EWL (Excess Weight Loss)A way to measure weight loss relative to how much you had to lose, expressed as a percentage. Losing half of your excess weight = 50% EWL. Common in GLP-1 research.FFMI (Fat-Free Mass Index)Like BMI but for muscle and lean tissue. It tells you how much of your weight is functional mass rather than fat. Useful for tracking whether you are preserving muscle during weight loss.GLP-1GLP-1 stands for glucagon-like peptide-1. GLP-1 medications (like Ozempic and Mounjaro) mimic a natural gut hormone that slows digestion and reduces appetite. They are injected weekly or daily depending on the medication.Goal weightThe target weight you set during onboarding. DayByDay uses this to calculate your progress percentage and to estimate when you might reach it based on your current trend.Journey ModeA premium dark/sunset theme in DayByDay that makes checking in feel like a reward. It changes the color palette to warm sunset tones and adjusts certain UI elements for a more atmospheric experience.Lean mass / Fat-free massThe part of your body weight that is not fat — including muscle, bone, water, and organs. Preserving lean mass while losing fat is the goal of most weight loss programs.Metabolic ageAn estimate of how old your metabolism is compared to population averages. A metabolic age lower than your actual age generally indicates a faster metabolism and healthier body composition.O2 RingA finger-worn device made by Wellue/Viatom that tracks blood oxygen (SpO2) and heart rate continuously overnight. DayByDay can import data from the O2Ring app to give you detailed overnight oxygen analysis.O2 ScoreDayByDay's summary score for your overnight blood oxygen quality. It factors in your average SpO2, how many oxygen dips you had, and your lowest recorded level. Higher is better.REM sleepRapid Eye Movement sleep — the stage where most dreaming happens. REM is critical for emotional regulation, memory, and learning. You cycle through REM several times a night, with longer periods toward morning.RFM (Relative Fat Mass)A body fat estimate calculated from your height and waist circumference. More accurate than BMI for most people because it accounts for where fat is actually distributed on your body.Resting heart rateYour heart rate when you are fully at rest — typically measured in the morning before getting out of bed. A lower resting heart rate is generally associated with better cardiovascular fitness.SemaglutideThe active ingredient in Ozempic (for diabetes) and Wegovy (for weight loss). It is a weekly injection that mimics the GLP-1 hormone to reduce appetite and slow digestion.SpO2Blood oxygen saturation — the percentage of your red blood cells that are carrying oxygen. A healthy level at rest is typically 95% or above. During sleep, brief dips are normal, but frequent drops can signal breathing issues.StreakThe number of consecutive days you have logged at least one data point in DayByDay. Streaks are shown on your dashboard as a motivational indicator of consistency.TDEE (Total Daily Energy Expenditure)The total number of calories your body burns in a day, including both your resting metabolism and all physical activity. DayByDay estimates this by applying an activity multiplier to your BMR.TirzepatideThe active ingredient in Mounjaro (for diabetes) and Zepbound (for weight loss). It is a weekly injection that targets both GLP-1 and GIP receptors, making it a dual-action medication.TitrationThe process of gradually increasing your medication dose over time, usually in steps of several weeks. GLP-1 medications are titrated slowly to minimize side effects like nausea.Trend lineA smoothed line on your weight chart that shows the direction your weight is moving, filtering out daily noise. DayByDay calculates this as a 7-day rolling average of your weigh-ins.WHtR (Waist-to-Height Ratio)Your waist circumference divided by your height. A ratio under 0.5 is generally considered healthy for most adults. It is a simple way to gauge whether abdominal fat is at a concerning level.