Hypoglycemia during sleep impairs consolidation of declarative memory in type 1 diabetic and healthy humans

OBJECTIVE - Early nocturnal sleep enhances the consolidation of declarative memories acquired during prior wakefulness. Patients with type 1 diabetes frequently experience hypoglycemic episodes during sleep. We investigated whether short-lasting hypoglycemia during early nocturnal sleep affects the sleep-associated consolidation of declarative memories. RESEARCH DESIGN AND METHODS - Sixteen type 1 diabetic patients and 16 healthy subjects matched for age and BMI were tested. On one condition, a linear fall of plasma glucose to 2.2 mmol/l was induced within 60 min by infusing insulin during early sleep. On the control condition, euglycemia (>3.86 mmol/l) was maintained throughout the night. In the morning, subjects recalled word pairs learned in the preceding evening. To assess mood and attention, a symptom questionnaire, an adjective check list, and the Stroop test were applied. Also, auditory event-related brain potentials were recorded. RESULTS - After euglycemia, subjects recalled 1.5 ± 0.5 more word pairs than after hypoglycemia (P < 0.01), remembering 2.0 ± 0.6 more word pairs than at immediate recall before sleep (P = 0.002). Across the hypoglycemic night, no such gain occurred (+0.5 ± 0.6 words; P = 0.41). Hypoglycemia during sleep also impaired mood (P < 0.05) but did not affect attention. Effects compared well between type 1 diabetic patients and healthy control subjects. CONCLUSIONS - Our findings indicate specific sensitivity of declarative memory consolidation during sleep to rather short episodes of mild hypoglycemia. This effect may disable memory processing in type 1 diabetic patients prone to nocturnal hypoglycemic episodes and underlines the importance of considering sleep as a critical period in the treatment of these patients.