diff --git a/src/runtime/runtime_nrf.c b/src/runtime/runtime_nrf.c
index 2271bf25..b0be67b5 100644
--- a/src/runtime/runtime_nrf.c
+++ b/src/runtime/runtime_nrf.c
@@ -17,6 +17,35 @@ void uart_send(uint8_t c) {
 	NRF_UART0->EVENTS_TXDRDY = 0;
 }
 
+void rtc_init() {
+	// Make sure the low-frequency clock is running.
+	NRF_CLOCK->TASKS_LFCLKSTART = 1;
+	while (NRF_CLOCK->EVENTS_LFCLKSTARTED == 0) {}
+	NRF_CLOCK->EVENTS_LFCLKSTARTED = 0;
+
+	NRF_RTC0->TASKS_START = 1;
+	NVIC_SetPriority(RTC0_IRQn, 3);
+	NVIC_EnableIRQ(RTC0_IRQn);
+}
+
+static volatile bool rtc_wakeup;
+
+void rtc_sleep(uint32_t ticks) {
+	NRF_RTC0->INTENSET = RTC_INTENSET_COMPARE0_Msk;
+	rtc_wakeup = false;
+	NRF_RTC0->TASKS_CLEAR = 1;
+	NRF_RTC0->CC[0] = ticks;
+	while (!rtc_wakeup) {
+		__WFI();
+	}
+}
+
+void RTC0_IRQHandler() {
+	NRF_RTC0->INTENCLR = RTC_INTENSET_COMPARE0_Msk;
+	NRF_RTC0->EVENTS_COMPARE[0] = 0;
+	rtc_wakeup = true;
+}
+
 void _start() {
 	main();
 }
diff --git a/src/runtime/runtime_nrf.go b/src/runtime/runtime_nrf.go
index 447ad28d..cb3d1550 100644
--- a/src/runtime/runtime_nrf.go
+++ b/src/runtime/runtime_nrf.go
@@ -8,6 +8,7 @@ import "C"
 
 func init() {
 	C.uart_init(6) // pin_tx = 6, for NRF52840-DK
+	C.rtc_init()
 }
 
 const Microsecond = 1
@@ -17,7 +18,7 @@ func putchar(c byte) {
 }
 
 func Sleep(d Duration) {
-	// TODO
+	C.rtc_sleep(C.uint32_t(d / 32)) // TODO: not accurate (must be d / 30.5175...)
 }
 
 func abort() {
diff --git a/src/runtime/runtime_nrf.h b/src/runtime/runtime_nrf.h
index 79c25806..61acc3f5 100644
--- a/src/runtime/runtime_nrf.h
+++ b/src/runtime/runtime_nrf.h
@@ -5,3 +5,6 @@
 
 void uart_init(uint32_t pin_tx);
 void uart_send(uint8_t c);
+
+void rtc_init();
+void rtc_sleep(uint32_t ticks);