# Reading a CGM without fooling yourself Fourteen things that move a sensor’s reading without a mouthful of food: lag, the first day, exercise, stress, a short night, illness, site, compression, heat, failures, interference, alcohol, the cycle, and one spike. Type: practice · Domain: Metabolic Health · Accuracy: Measured · Importance: Core Also called: CGM gotchas, CGM accuracy, why is my CGM wrong, compression low, sensor lag, false low, CGM mistakes, first day readings ## Do this - Compare sensor with sensor, never sensor with finger-prick. - Ignore the first day. - Log alcohol, training, late meals, sleep and sick days, because they are the explanations. - Keep the sensor on the same arm. - Sleep on the other side. - Repeat a meal before you judge it. - Read the month, not the morning. ## Before you blame a meal Fourteen things a sensor does that are not about your diet. Each is one short paragraph, with its source where one exists; where a point rests on VT's own practice or on the sensor's label, it says so. ## 1. It lags The sensor reads the fluid between cells, which follows the blood. Measured directly with tracers in eight healthy adults in the fasted state, glucose took five to six minutes to cross into the tissue fluid (Basu 2013); after a meal, when glucose is rising fast, the sensor runs further behind, and after exercise it can be ahead or behind. So a sensor's peak is later and often a little lower than blood's, and a finger-prick taken while glucose is moving will disagree with it. Compare sensor with sensor. ## 2. The first day is rough Readings in the first 12 to 24 hours after insertion can run low or erratic while the tissue around the filament settles. VT keeps the one-hour warm-up out of every figure and marks the first day in every summary; if the first day looks nothing like the rest, that is why. (VT practice, from the sensor's documented warm-up and common experience; no study is cited for the first-day effect.) ## 3. Exercise can raise glucose Hard intervals and heavy lifting release adrenaline and stored glucose from the liver faster than the muscles take it up, so a spike during a session is physiology, not breakfast; steady, easy sessions usually lower glucose. The consensus on exercise in type 1 diabetes sets this out in detail, with high-intensity and anaerobic work raising glucose and aerobic work lowering it (Riddell 2017); the physiology is the same without diabetes, in a smaller range. ## 4. Stress raises it too A hard meeting, an argument or a fright moves glucose through the same hormones as hard exercise, without a mouthful of food. The daily stress question and the notes are where the explanation lives. (General physiology; no study is cited.) ## 5. A short night costs the next day A single night of four hours' sleep cut the glucose nine healthy adults could dispose of under a clamp by about a quarter the next day, with the liver releasing more and the muscles taking up less (Donga 2010); six nights of four hours lowered glucose tolerance in young men to a degree the authors compared with normal aging (Spiegel 1999). Higher readings after the same meals, and a higher daily mean, after a bad night are the clearest link in the record, and the reason VT draws glucose beside sleep. ## 6. Being unwell changes everything A fever or an infection raises glucose for days through the same stress hormones, and the sick days are not your baseline. VT keeps sick days and the three after out of your Vitality Score's glucose carry-forward and out of the comparisons; only you mark them. (General physiology for the rise; VT practice for the rule.) ## 7. Where it sits changes what it reads For some people the left and right arms read differently by a steady amount, sometimes a whole mmol/L, and the abdomen differs again. Keep the site when you can; when you change it, expect the level to shift and compare within a site. VT records the site of every sensor so that the difference is visible rather than mysterious. (VT's own observation across members; no study is cited.) ## 8. Lying on it lowers it Pressure on the sensor in sleep squeezes the tissue it reads and produces a sudden false low. In healthy volunteers wearing four abdominal sensors each for four nights, recorded on video, individual sensors intermittently gave readings more than 25 mg/dL (1.4 mmol/L) away from the median of the four, and those readings were strongly correlated with the person lying on that sensor; most were sudden drops (Mensh 2013). VT marks nights where the pattern appears and takes the readings out of your minutes below 3.9; the fix is to sleep on the other side. ## 9. Heat and dehydration move it A hot bath, a sauna or a long hot day changes blood flow under the skin, and a dehydrated afternoon changes the fluid the sensor sits in; readings in those hours are less trustworthy than the rest. Log the sauna and read around it. (General physiology; no study is cited.) ## 10. Sensors fail Readings can drift, stick or jump before a sensor dies. VT catches impossible values and sudden steps that the body cannot produce, takes them out of your averages and tells you; a sensor that fails before its 14 days is replaced by the manufacturer, and the store page says how. ## 11. Some medicines and vitamins can interfere, with some sensors The chemistry at the filament's tip can be confused by other molecules. Several sensors' labels warn that high-dose vitamin C or certain painkillers shift their readings; the GS3's documentation lists no such interference. Your sensor's own label is the authority on this, not this page: read it when you start a new supplement. ## 12. Alcohol lowers overnight glucose The liver gives alcohol priority. In men given 48 grams of alcohol after an overnight fast, the liver's production of new glucose fell by 45 percent over the following five hours (Siler 1998); in those fasted men other routes made up the difference and blood glucose held, but across a whole night after drinks with little food, the sensor often shows a lower, flatter line and sometimes minutes below 3.9. Logged alcohol explains it, and VT reads the night with the log. ## 13. The cycle moves it In the second half of the menstrual cycle insulin sensitivity falls for many women, and the same meals read higher. It is seen most clearly where it can be measured most precisely, in women with type 1 diabetes: insulin sensitivity measured by clamp was lower in the luteal phase than the follicular in 16 women (Diaz 2022), and over 33 cycles in 12 women, sensor glucose ran higher around ovulation and the early luteal phase with insulin sensitivity depressed through the luteal phase (Brown 2015). Read a luteal week against other luteal weeks, not against the week before. ## 14. One spike is not a verdict The same meal in the same person varies from day to day, and between people it varies enormously (Zeevi 2015; Berry 2020). Compare a meal on its two-hour iAUC, eat it again on another day, and only then draw a conclusion; VT's meal comparison does the repetition for you and shows the repeats together. ## Sources 1. Basu A, et al. Time lag of glucose from intravascular to interstitial compartment in humans. Diabetes 2013;62:4083–7. [doi:10.2337/db13-1132](https://doi.org/10.2337/db13-1132) 2. Riddell MC, et al. Exercise management in type 1 diabetes: a consensus statement. Lancet Diabetes Endocrinol 2017;5:377–90. [doi:10.1016/S2213-8587(17)30014-1]() 3. Donga E, et al. J Clin Endocrinol Metab 2010;95:2963–8. [doi:10.1210/jc.2009-2430](https://doi.org/10.1210/jc.2009-2430) 4. Spiegel K, Leproult R, Van Cauter E. Lancet 1999;354:1435–9. [doi:10.1016/S0140-6736(99)01376-8]() 5. Mensh BD, Wisniewski NA, Neil BM, Burnett DR. Susceptibility of interstitial continuous glucose monitor performance to sleeping position. J Diabetes Sci Technol 2013;7:863–70. [doi:10.1177/193229681300700408](https://doi.org/10.1177/193229681300700408) 6. Siler SQ, Neese RA, Christiansen MP, Hellerstein MK. The inhibition of gluconeogenesis following alcohol in humans. Am J Physiol 1998;275:E897–907. [doi:10.1152/ajpendo.1998.275.5.E897](https://doi.org/10.1152/ajpendo.1998.275.5.E897) 7. Diaz C JL, Fabris C, Breton MD, Cengiz E. Insulin replacement across the menstrual cycle in women with type 1 diabetes. Diabetes Technol Ther 2022;24:832–41. [doi:10.1089/dia.2022.0154](https://doi.org/10.1089/dia.2022.0154) 8. Brown SA, et al. Fluctuations of hyperglycemia and insulin sensitivity are linked to menstrual cycle phases in women with T1D. J Diabetes Sci Technol 2015;9:1192–9. [doi:10.1177/1932296815608400](https://doi.org/10.1177/1932296815608400) 9. Zeevi D, et al. Cell 2015;163:1079–94. [doi:10.1016/j.cell.2015.11.001](https://doi.org/10.1016/j.cell.2015.11.001) 10. Berry SE, et al. Nat Med 2020;26:964–73. [doi:10.1038/s41591-020-0934-0](https://doi.org/10.1038/s41591-020-0934-0) 11. Sibionics GS3 product page and documentation, read 19 October 2026 12. Vitality Tracker, CGM page brief (19 October 2026). ## Related - [What a CGM measures](https://vitalitytracker.app/knowledge/what-a-cgm-measures) - [Sensor coverage](https://vitalitytracker.app/knowledge/sensor-coverage) - [Time above and below](https://vitalitytracker.app/knowledge/time-above-and-below) - [Peak rise](https://vitalitytracker.app/knowledge/peak-rise) - [Two-hour incremental area under the curve](https://vitalitytracker.app/knowledge/two-hour-iauc) - [Sick days, holidays and unusual periods](https://vitalitytracker.app/knowledge/sick-days-holidays-and-unusual-periods) - [Protocols: log the things you do to yourself](https://vitalitytracker.app/knowledge/protocols-how-to) Reviewed October 2026 · v1 Source: https://vitalitytracker.app/knowledge/reading-a-cgm-without-fooling-yourself