The blood sugar level is the amount of glucose (sugar) in the blood. It is also known as serum glucose level. It is expressed as millimoles per litre (mmol/l). Normally, blood glucose levels stay within narrow limits throughout the day: 4 to 8mmol/l. But they are higher after meals and usually lowest in the morning. In diabetes the blood sugar level moves outside these limits until treated. Even with good control of diabetes, the blood sugar level will still at times drift outside this normal range.