diff --git a/.gitignore b/.gitignore
index 4fffb2f..8eb581d 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,2 +1,3 @@
 /target
 /Cargo.lock
+/.idea
diff --git a/backon/Cargo.toml b/backon/Cargo.toml
index e83feb9..110db12 100644
--- a/backon/Cargo.toml
+++ b/backon/Cargo.toml
@@ -13,10 +13,10 @@ repository.workspace = true
 [package.metadata.docs.rs]
 all-features = true
 targets = [
-  "x86_64-unknown-linux-gnu",
-  "x86_64-apple-darwin",
-  "x86_64-pc-windows-msvc",
-  "wasm32-unknown-unknown",
+    "x86_64-unknown-linux-gnu",
+    "x86_64-apple-darwin",
+    "x86_64-pc-windows-msvc",
+    "wasm32-unknown-unknown",
 ]
 
 [features]
@@ -37,21 +37,22 @@ gloo-timers = { version = "0.3", optional = true }
 [dev-dependencies]
 anyhow = "1"
 reqwest = "0.12"
+spin = "0.9.8"
 
 [target.'cfg(target_arch = "wasm32")'.dev-dependencies]
 tokio = { version = "1", features = [
-  "macros",
-  "rt",
-  "sync",
+    "macros",
+    "rt",
+    "sync",
 ], default-features = false }
 wasm-bindgen-test = "0.3"
 
 [target.'cfg(not(target_arch = "wasm32"))'.dev-dependencies]
 sqlx = { version = "0.8.0", features = ["runtime-tokio", "sqlite"] }
 tokio = { version = "1", features = [
-  "time",
-  "rt",
-  "macros",
-  "sync",
-  "rt-multi-thread",
+    "time",
+    "rt",
+    "macros",
+    "sync",
+    "rt-multi-thread",
 ] }
diff --git a/backon/src/backoff/api.rs b/backon/src/backoff/api.rs
index 4791cc8..3f8994c 100644
--- a/backon/src/backoff/api.rs
+++ b/backon/src/backoff/api.rs
@@ -1,5 +1,5 @@
-use std::fmt::Debug;
-use std::time::Duration;
+use core::fmt::Debug;
+use core::time::Duration;
 
 /// BackoffBuilder is utilized to construct a new backoff.
 pub trait BackoffBuilder: Debug + Send + Sync + Unpin {
diff --git a/backon/src/backoff/constant.rs b/backon/src/backoff/constant.rs
index 46a8963..749c7bf 100644
--- a/backon/src/backoff/constant.rs
+++ b/backon/src/backoff/constant.rs
@@ -1,4 +1,4 @@
-use std::time::Duration;
+use core::time::Duration;
 
 use crate::backoff::BackoffBuilder;
 
@@ -121,7 +121,7 @@ impl Iterator for ConstantBackoff {
 
 #[cfg(test)]
 mod tests {
-    use std::time::Duration;
+    use core::time::Duration;
 
     #[cfg(target_arch = "wasm32")]
     use wasm_bindgen_test::wasm_bindgen_test as test;
diff --git a/backon/src/backoff/exponential.rs b/backon/src/backoff/exponential.rs
index a08ab66..a292452 100644
--- a/backon/src/backoff/exponential.rs
+++ b/backon/src/backoff/exponential.rs
@@ -1,4 +1,4 @@
-use std::time::Duration;
+use core::time::Duration;
 
 use crate::backoff::BackoffBuilder;
 
@@ -182,7 +182,7 @@ pub(crate) fn saturating_mul(d: Duration, rhs: f32) -> Duration {
 
 #[cfg(test)]
 mod tests {
-    use std::time::Duration;
+    use core::time::Duration;
 
     #[cfg(target_arch = "wasm32")]
     use wasm_bindgen_test::wasm_bindgen_test as test;
diff --git a/backon/src/backoff/fibonacci.rs b/backon/src/backoff/fibonacci.rs
index 81ed294..2137c0b 100644
--- a/backon/src/backoff/fibonacci.rs
+++ b/backon/src/backoff/fibonacci.rs
@@ -1,4 +1,4 @@
-use std::time::Duration;
+use core::time::Duration;
 
 use crate::backoff::BackoffBuilder;
 
@@ -164,7 +164,7 @@ impl Iterator for FibonacciBackoff {
 
 #[cfg(test)]
 mod tests {
-    use std::time::Duration;
+    use core::time::Duration;
 
     #[cfg(target_arch = "wasm32")]
     use wasm_bindgen_test::wasm_bindgen_test as test;
diff --git a/backon/src/blocking_retry.rs b/backon/src/blocking_retry.rs
index cb901ce..8194f5e 100644
--- a/backon/src/blocking_retry.rs
+++ b/backon/src/blocking_retry.rs
@@ -1,4 +1,4 @@
-use std::time::Duration;
+use core::time::Duration;
 
 use crate::backoff::BackoffBuilder;
 use crate::blocking_sleep::MaybeBlockingSleeper;
@@ -183,7 +183,7 @@ where
     /// # Examples
     ///
     /// ```no_run
-    /// use std::time::Duration;
+    /// use core::time::Duration;
     ///
     /// use anyhow::Result;
     /// use backon::BlockingRetryable;
@@ -229,7 +229,7 @@ where
 {
     /// Call the retried function.
     ///
-    /// TODO: implement [`std::ops::FnOnce`] after it stable.
+    /// TODO: implement [`FnOnce`] after it stable.
     pub fn call(mut self) -> Result<T, E> {
         loop {
             let result = (self.f)();
@@ -255,8 +255,11 @@ where
 }
 #[cfg(test)]
 mod tests {
-    use std::sync::Mutex;
-    use std::time::Duration;
+    use alloc::string::ToString;
+    use alloc::vec;
+    use alloc::vec::Vec;
+    use core::time::Duration;
+    use spin::Mutex;
 
     use super::*;
     use crate::ExponentialBuilder;
@@ -281,7 +284,7 @@ mod tests {
         let error_times = Mutex::new(0);
 
         let f = || {
-            let mut x = error_times.lock().unwrap();
+            let mut x = error_times.lock();
             *x += 1;
             Err::<(), anyhow::Error>(anyhow::anyhow!("not retryable"))
         };
@@ -297,7 +300,7 @@ mod tests {
         assert_eq!("not retryable", result.unwrap_err().to_string());
         // `f` always returns error "not retryable", so it should be executed
         // only once.
-        assert_eq!(*error_times.lock().unwrap(), 1);
+        assert_eq!(*error_times.lock(), 1);
         Ok(())
     }
 
@@ -306,8 +309,8 @@ mod tests {
         let error_times = Mutex::new(0);
 
         let f = || {
-            println!("I have been called!");
-            let mut x = error_times.lock().unwrap();
+            // println!("I have been called!");
+            let mut x = error_times.lock();
             *x += 1;
             Err::<(), anyhow::Error>(anyhow::anyhow!("retryable"))
         };
@@ -323,7 +326,7 @@ mod tests {
         assert_eq!("retryable", result.unwrap_err().to_string());
         // `f` always returns error "retryable", so it should be executed
         // 4 times (retry 3 times).
-        assert_eq!(*error_times.lock().unwrap(), 4);
+        assert_eq!(*error_times.lock(), 4);
         Ok(())
     }
 
diff --git a/backon/src/blocking_retry_with_context.rs b/backon/src/blocking_retry_with_context.rs
index 1bb1db1..e66fa46 100644
--- a/backon/src/blocking_retry_with_context.rs
+++ b/backon/src/blocking_retry_with_context.rs
@@ -1,4 +1,4 @@
-use std::time::Duration;
+use core::time::Duration;
 
 use crate::backoff::BackoffBuilder;
 use crate::blocking_sleep::MaybeBlockingSleeper;
@@ -152,7 +152,7 @@ where
 {
     /// Call the retried function.
     ///
-    /// TODO: implement [`std::ops::FnOnce`] after it stable.
+    /// TODO: implement [`FnOnce`] after it stable.
     pub fn call(mut self) -> (Ctx, Result<T, E>) {
         let mut ctx = self.ctx.take().expect("context must be valid");
         loop {
@@ -182,14 +182,13 @@ where
 
 #[cfg(test)]
 mod tests {
-    use std::time::Duration;
-
-    use anyhow::anyhow;
-    use std::sync::Mutex;
-
     use super::*;
     use crate::ExponentialBuilder;
+    use alloc::string::ToString;
+    use anyhow::anyhow;
     use anyhow::Result;
+    use core::time::Duration;
+    use spin::Mutex;
 
     struct Test;
 
@@ -209,7 +208,7 @@ mod tests {
 
         let (_, result) = {
             |mut v: Test| {
-                let mut x = error_times.lock().unwrap();
+                let mut x = error_times.lock();
                 *x += 1;
 
                 let res = v.hello();
@@ -226,7 +225,7 @@ mod tests {
         assert_eq!("not retryable", result.unwrap_err().to_string());
         // `f` always returns error "not retryable", so it should be executed
         // only once.
-        assert_eq!(*error_times.lock().unwrap(), 1);
+        assert_eq!(*error_times.lock(), 1);
         Ok(())
     }
 }
diff --git a/backon/src/blocking_sleep.rs b/backon/src/blocking_sleep.rs
index 72b8ebe..5f5c09d 100644
--- a/backon/src/blocking_sleep.rs
+++ b/backon/src/blocking_sleep.rs
@@ -1,4 +1,4 @@
-use std::time::Duration;
+use core::time::Duration;
 
 /// A sleeper is used sleep for a specified duration.
 pub trait BlockingSleeper: 'static {
@@ -24,7 +24,7 @@ impl<F: Fn(Duration) + 'static> BlockingSleeper for F {
 /// The default implementation of `Sleeper` when no features are enabled.
 ///
 /// It will fail to compile if a containing [`Retry`][crate::Retry] is `.await`ed without calling [`Retry::sleep`][crate::Retry::sleep] to provide a valid sleeper.
-#[cfg(all(not(feature = "tokio-sleep"), not(feature = "gloo-timers-sleep")))]
+#[cfg(not(feature = "std-blocking-sleep"))]
 pub type DefaultBlockingSleeper = PleaseEnableAFeatureOrProvideACustomSleeper;
 /// The default implementation of `Sleeper` while feature `std-blocking-sleep` enabled.
 ///
diff --git a/backon/src/lib.rs b/backon/src/lib.rs
index 917be53..a318cd2 100644
--- a/backon/src/lib.rs
+++ b/backon/src/lib.rs
@@ -52,7 +52,7 @@
 //! use anyhow::Result;
 //! use backon::ExponentialBuilder;
 //! use backon::Retryable;
-//! use std::time::Duration;
+//! use core::time::Duration;
 //!
 //! async fn fetch() -> Result<String> {
 //!     Ok("hello, world!".to_string())
@@ -84,7 +84,7 @@
 //! use anyhow::Result;
 //! use backon::BlockingRetryable;
 //! use backon::ExponentialBuilder;
-//! use std::time::Duration;
+//! use core::time::Duration;
 //!
 //! fn fetch() -> Result<String> {
 //!     Ok("hello, world!".to_string())
@@ -109,6 +109,12 @@
 
 #![deny(missing_docs)]
 #![deny(unused_qualifications)]
+#![no_std]
+
+#[cfg(feature = "std-blocking-sleep")]
+extern crate std;
+
+extern crate alloc;
 
 mod backoff;
 pub use backoff::*;
@@ -130,12 +136,10 @@ pub use sleep::Sleeper;
 pub use sleep::TokioSleeper;
 
 mod blocking_retry;
-pub use blocking_retry::BlockingRetry;
-pub use blocking_retry::BlockingRetryable;
+pub use blocking_retry::{BlockingRetry, BlockingRetryable};
 
 mod blocking_retry_with_context;
-pub use blocking_retry_with_context::BlockingRetryWithContext;
-pub use blocking_retry_with_context::BlockingRetryableWithContext;
+pub use blocking_retry_with_context::{BlockingRetryWithContext, BlockingRetryableWithContext};
 
 mod blocking_sleep;
 pub use blocking_sleep::BlockingSleeper;
diff --git a/backon/src/retry.rs b/backon/src/retry.rs
index b70e5fc..e084139 100644
--- a/backon/src/retry.rs
+++ b/backon/src/retry.rs
@@ -1,9 +1,9 @@
-use std::future::Future;
-use std::pin::Pin;
-use std::task::ready;
-use std::task::Context;
-use std::task::Poll;
-use std::time::Duration;
+use core::future::Future;
+use core::pin::Pin;
+use core::task::ready;
+use core::task::Context;
+use core::task::Poll;
+use core::time::Duration;
 
 use crate::backoff::BackoffBuilder;
 use crate::sleep::MaybeSleeper;
@@ -202,7 +202,7 @@ where
     /// # Examples
     ///
     /// ```no_run
-    /// use std::time::Duration;
+    /// use core::time::Duration;
     ///
     /// use anyhow::Result;
     /// use backon::ExponentialBuilder;
@@ -321,7 +321,10 @@ where
 #[cfg(test)]
 #[cfg(any(feature = "tokio-sleep", feature = "gloo-timers-sleep"))]
 mod default_sleeper_tests {
-    use std::time::Duration;
+    use alloc::string::ToString;
+    use alloc::vec;
+    use alloc::vec::Vec;
+    use core::time::Duration;
     use tokio::sync::Mutex;
 
     #[cfg(target_arch = "wasm32")]
@@ -429,7 +432,8 @@ mod default_sleeper_tests {
 
 #[cfg(test)]
 mod custom_sleeper_tests {
-    use std::{future::ready, time::Duration};
+    use alloc::string::ToString;
+    use core::{future::ready, time::Duration};
 
     #[cfg(target_arch = "wasm32")]
     use wasm_bindgen_test::wasm_bindgen_test as test;
diff --git a/backon/src/retry_with_context.rs b/backon/src/retry_with_context.rs
index 4452fbd..2eba93d 100644
--- a/backon/src/retry_with_context.rs
+++ b/backon/src/retry_with_context.rs
@@ -1,9 +1,9 @@
-use std::future::Future;
-use std::pin::Pin;
-use std::task::ready;
-use std::task::Context;
-use std::task::Poll;
-use std::time::Duration;
+use core::future::Future;
+use core::pin::Pin;
+use core::task::ready;
+use core::task::Context;
+use core::task::Poll;
+use core::time::Duration;
 
 use crate::backoff::BackoffBuilder;
 use crate::sleep::MaybeSleeper;
@@ -242,7 +242,7 @@ where
     /// # Examples
     ///
     /// ```no_run
-    /// use std::time::Duration;
+    /// use core::time::Duration;
     ///
     /// use anyhow::Result;
     /// use backon::ExponentialBuilder;
@@ -364,9 +364,9 @@ where
 #[cfg(test)]
 #[cfg(any(feature = "tokio-sleep", feature = "gloo-timers-sleep"))]
 mod tests {
-    use std::time::Duration;
-
+    use alloc::string::ToString;
     use anyhow::{anyhow, Result};
+    use core::time::Duration;
     use tokio::sync::Mutex;
 
     #[cfg(target_arch = "wasm32")]
diff --git a/backon/src/sleep.rs b/backon/src/sleep.rs
index 0aaeb94..4a5ab46 100644
--- a/backon/src/sleep.rs
+++ b/backon/src/sleep.rs
@@ -1,4 +1,4 @@
-use std::{
+use core::{
     future::{Future, Ready},
     time::Duration,
 };