Blood Sugar 66 mg/dl — Too Low? What It Means

Waking up at 66: clues that the night went low

A reading of 66 mg/dl first thing in the morning is below the 70 line, so it counts as a low and should be treated before breakfast plans. It also raises a question: was glucose even lower earlier in the night? Lows during sleep often go unnoticed, because sleep dulls the warning signs that would wake most people during the day.

The body leaves traces, though. Many people only find out about night lows by learning to read those traces.

Morning signs worth taking seriously

• Damp pajamas or sweat-soaked sheets on a night that was not hot.
• A headache on waking that is not explained by alcohol, caffeine withdrawal or poor sleep.
• Nightmares or vivid, restless dreams, sometimes with shouting or thrashing that a partner notices.
• Feeling unusually tired, groggy or “hungover” despite a normal night’s sleep.
• A partner’s report of fast breathing, trembling, or being hard to wake.

None of these proves a low on its own. Together with a waking reading in the 60s, they make a night-time low quite likely.

Some signs are easy to dismiss. A dry mouth or a racing heart on waking, or breakfast numbers that jump around from day to day for no clear reason, can also hint that the night was unsettled. Writing these down next to the morning reading for a week often shows a link that is invisible from a single day.

What tends to cause night lows

For people who use insulin or a sulfonylurea, the common causes are an evening dose that is too strong for the night, a smaller or earlier dinner than usual, exercise in the afternoon or evening, and alcohol. Long-acting insulin taken at bedtime, or mealtime insulin given to correct a high late in the evening, can also tip the balance.

A night low is more often followed by a lower-than-usual morning reading, though some people wake high after treating or eating during the night without remembering it. That is why a single fasting number is a poor guide to what happened at 2 or 3 a.m. For more on why morning numbers move, the page on the dawn phenomenon covers the opposite problem.

How to catch a night low on the record

The simplest tool is a meter check at about 3 a.m. on two or three ordinary nights, plus a check at bedtime. If you have a continuous glucose monitor, its overnight graph shows the whole night, and setting a low alert can wake you before you get too low.

When you check at night, also note what you ate for dinner, when your last insulin dose was given, any alcohol and any exercise that day. Night lows on days with afternoon sport but not on rest days, for example, point to a clear fix.

Take the bedtime, night and waking readings to your care team. Night lows are usually fixable by moving or changing a dose, adjusting the evening meal, or adding a bedtime snack on exercise days. They matter beyond comfort: repeated lows during sleep can weaken the warning signs during the day as well.

Keep fast sugar and your meter within reach of the bed. If you live alone and use insulin, it is worth asking about a CGM that can share alerts with a family member.