The first morning urine sample is recommended because it tends to be more concentrated, relatively free of dietary influences, and changes due to hydration and physical activity, thus abnormalities are more easily detected. The first morning urine is more concentrated because of the way the body regulates fluids and metabolism during sleep:
Overnight Fluid Restriction
- While you’re asleep, you typically don’t drink water for 6–10 hours.
- The kidneys continue to filter blood during this time, but with no incoming fluids, the volume of urine produced is smaller, and solutes (like salts, urea, and metabolites) become more concentrated.
Hormonal Regulation
- At night, the body produces more antidiuretic hormone (ADH, or vasopressin).
- ADH signals the kidneys to conserve water, resulting in less urine volume and higher concentration.
- This mechanism helps prevent dehydration while you sleep.
Metabolic Waste Accumulation
- During sleep, cellular processes still produce waste (urea, creatinine, metabolites from detox pathways).
- Since urine output is low, these solutes build up in the bladder overnight, making the first morning urine sample more concentrated in biomarkers.
Clinical and Testing Relevance
- Because first morning urine is the most concentrated sample, it’s often preferred for lab testing (e.g., pregnancy tests, drug screening, metabolic and nutritional tests).
- It provides a stronger signal for analytes that might be diluted later in the day when you’re drinking fluids regularly.
Waking in Middle of Night
- If you wake in the middle of the night, do not collect urine. Return to sleep and collect urine at the first morning waking.